[202012][DellEMC] Z9332f PSU data is not updated in state DB (#7543)

#### Why I did it

- PSU data is loaded into state DB.
   Following errors are seen in syslogs:
  "Failed to update PSU data - '<=' not supported between instances of 'float' and 'str'"

- Issue is not seen in master image as the PSU API return type is different.

#### How I did it

- Changed the return type in PSU API's.

platform/broadcom/sonic-platform-modules-dell/z9332f/sonic_platform/psu.py

@@ -113,7 +113,7 @@ def get_voltage(self):
         if not is_valid:
             return None
 
-        return "{:.1f}".format(voltage)
+        return float(voltage)
 
     def get_voltage_low_threshold(self):
         """
@@ -159,7 +159,7 @@ def get_current(self):
         if not is_valid:
             return None
 
-        return "{:.1f}".format(current)
+        return float(current)
 
     def get_power(self):
         """
@@ -173,7 +173,7 @@ def get_power(self):
         if not is_valid:
             return None
 
-        return "{:.1f}".format(power)
+        return float(power)
 
     def get_powergood_status(self):
         """