clang-tidy: fix some warnings

Resolve the following:
```
sys_info_param.cpp:12:11: style: Variable 'callback' can be declared as reference to const [constVariableReference]
    auto& callback = iterator->second;
          ^
include/ipmid/sessionhelper.hpp:49:31: performance: Ineffective call of function 'substr' because a prefix of the string is assigned to itself. Use resize() or pop_back() instead. [uselessCallsSubstr]
            sessionIdString = sessionIdString.substr(0, pos);
                              ^
include/ipmid/sessionhelper.hpp:36:14: style: The scope of the variable 'tempSessionHandle' can be reduced. [variableScope]
    uint16_t tempSessionHandle = 0;
             ^
include/ipmid/sessionhelper.hpp:77:47: performance: Function parameter 'objectPath' should be passed by const reference. [passedByValue]
bool isSessionObjectMatched(const std::string objectPath,
                                              ^
include/ipmid/sessionhelper.hpp:50:31: performance: Ineffective call of function 'substr' because a prefix of the string is assigned to itself. Use resize() or pop_back() instead. [uselessCallsSubstr]
            sessionIdString = sessionIdString.substr(0, pos);
                              ^
transport/serialbridge/test/serial_unittest.cpp:21:13: style: The scope of the variable 'c' can be reduced. [variableScope]
    uint8_t c;
            ^
```

Change-Id: I5a34fc50a95c33367e8b5d67dcb94c46b8def94a
Signed-off-by: George Liu <liuxiwei@ieisystem.com>
diff --git a/include/ipmid/sessionhelper.hpp b/include/ipmid/sessionhelper.hpp
index 5c09bfc..3afb5fa 100644
--- a/include/ipmid/sessionhelper.hpp
+++ b/include/ipmid/sessionhelper.hpp
@@ -33,7 +33,6 @@
     // getting the position of session id and session handle string from
     // object path.
     std::size_t ptrPosition = objectPath.rfind("/");
-    uint16_t tempSessionHandle = 0;
 
     if (ptrPosition != std::string::npos)
     {
@@ -47,10 +46,11 @@
             // extracting the session handle
             std::string sessionHandleString = sessionIdString.substr(pos + 1);
             // extracting the session id
-            sessionIdString = sessionIdString.substr(0, pos);
+            sessionIdString.resize(pos);
             // converting session id string  and session handle string to
             // hexadecimal.
             std::stringstream handle(sessionHandleString);
+            uint16_t tempSessionHandle = 0;
             handle >> std::hex >> tempSessionHandle;
             sessionHandle = tempSessionHandle & 0xFF;
             std::stringstream idString(sessionIdString);
@@ -74,7 +74,7 @@
  *
  * @return true if the object is matched else return false
  **/
-bool isSessionObjectMatched(const std::string objectPath,
+bool isSessionObjectMatched(const std::string& objectPath,
                             const uint32_t reqSessionId,
                             const uint8_t reqSessionHandle)
 {