iwlwifi: remove set_pwr_src operation

The set_pwr_src operation is only ever used from
within the same sub-driver that it is declared
in, so it can just be called directly instead of
being an operation. Also, it is never called to
set the power source to V_aux, so change the two
functions accordingly (but keep the V_aux code
for documentation purposes in a comment).

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>