Minor path/string op tweaks

Store the hwmon instance and hwmon classpath root
separately.

Use string::assign(char) for where appropriate.

Change-Id: I4ba756a262b9efee7a31610bf01c014974d27af3
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>
diff --git a/mainloop.hpp b/mainloop.hpp
index c5038f7..f48e590 100644
--- a/mainloop.hpp
+++ b/mainloop.hpp
@@ -61,8 +61,10 @@
         sdbusplus::server::manager::manager _manager;
         /** @brief Shutdown requested. */
         volatile bool _shutdown;
-        /** @brief Path to hwmon sysfs instance. */
-        std::string _path;
+        /** @brief hwmon sysfs class path. */
+        std::string _hwmonRoot;
+        /** @brief hwmon sysfs instance. */
+        std::string _instance;
         /** @brief DBus busname prefix. */
         const char* _prefix;
         /** @brief DBus sensors namespace root. */