psu-ng: Fixed PSU monitor app to bind/unbind device driver appropriately.

Modified bindOrUnbind function:
      Bind the  device driver when the PSU present and i2cbus-i2caddr
      does not exist. Unbind the device driver when i2cbus-i2caddr exists
      and the PSU not present.

      bindOrUnbind does not do anything when:
      * PSU device driver bind exist and the device present
      * PSU device driver bind does not exist and device not present

Added new function populateDriverName:
      Search for device driver name in all PSUs, then assign the device
      driver name to all PSUs including missing PSUs.

Testing:
    Verified in the following test, there is no bind/unbind error log.
        1 - Removed 1 PSU and powered on the BMC with 3 PSUs
        2 - Use step 1, after power on plugged in the 4th PSU and verified
            /sys/bus/i2c/drivers/-ibm-cffps/3-006b exist
        3 - Removed 2 PSUs and powered on the BMC with 2 PSUs.
        4 - Use step 3, after power on plugged in the missing PSUs, verified
            bind/Unbind /sys/bus/i2c/drivers/-ibm-cffps/3-006x for appropriate PSU.
        5 - Powered on system with 4 PSUs then removed 2 PSUs. Verified
            unbind done for appropriate PSU.

Change-Id: I325fc0dbb16f9c3b582b739e8440a74e34aae753
Signed-off-by: Faisal Awada <faisal@us.ibm.com>
diff --git a/phosphor-power-supply/power_supply.cpp b/phosphor-power-supply/power_supply.cpp
index fa4b8e3..6ec8115 100644
--- a/phosphor-power-supply/power_supply.cpp
+++ b/phosphor-power-supply/power_supply.cpp
@@ -96,9 +96,36 @@
 
 void PowerSupply::bindOrUnbindDriver(bool present)
 {
+    // Symbolic link to the device will exist if the driver is bound.
+    // So exit no action required if both the link and PSU are present
+    // or neither is present.
+    namespace fs = std::filesystem;
+    fs::path path;
     auto action = (present) ? "bind" : "unbind";
-    auto path = bindPath / action;
 
+    // This case should not happen, if no device driver name return.
+    if (driverName.empty())
+    {
+        log<level::INFO>("No device driver name found");
+        return;
+    }
+    if (bindPath.string().find(driverName) != std::string::npos)
+    {
+        // bindPath has driver name
+        path = bindPath / action;
+    }
+    else
+    {
+        // Add driver name to bindPath
+        path = bindPath / driverName / action;
+        bindPath = bindPath / driverName;
+    }
+
+    if ((std::filesystem::exists(bindPath / bindDevice) && present) ||
+        (!std::filesystem::exists(bindPath / bindDevice) && !present))
+    {
+        return;
+    }
     if (present)
     {
         std::this_thread::sleep_for(std::chrono::milliseconds(bindDelay));