[sfputil] Make SfpUtilHelper.get_physical_to_logical noexcept as in SfpUtilBase (sonic-net#96)

The SfpUtil interface used in xcvrd expects get_physical_to_logical to return None in the case of unknown port:
https://github.com/Azure/sonic-platform-daemons/blob/master/sonic-xcvrd/scripts/xcvrd#L906

Note SfpUtilBase implementation already conforms the interface in xcvrd:
https://github.com/Azure/sonic-platform-common/blob/master/sonic_platform_base/sonic_sfp/sfputilbase.py#L603

Signed-off-by: Volodymyr Boyko <[email protected]>

Author: vboykox

sonic_platform_base/sonic_sfp/sfputilhelper.py

@@ -172,7 +172,7 @@ def read_porttab_mappings(self, porttabfile):
     def get_physical_to_logical(self, port_num):
         """Returns list of logical ports for the given physical port"""
 
-        return self.physical_to_logical[port_num]
+        return self.physical_to_logical.get(port_num)
 
     def get_logical_to_physical(self, logical_port):
         """Returns list of physical ports for the given logical port"""