power: smb5-lib: Optimize PD voltage configuration flow

Currently, on every external call to set PD max and min voltage,
switcher frequency is reconfigured and power_supply_changed is
call to update the user-space daemons. However, this flow is
executed for every call irrespective of the voltage value being
configured, introducing redundant flows given same voltage level
is reconfigured. Optimize the configuration flow to skip same
voltage level configuration.

Change-Id: I9c2e2665b66418612ffacb679fed579142e18caf
Signed-off-by: Umang Agrawal <uagrawal@codeaurora.org>