iwlwifi: cleanup set_pwr_src

This patch cleans up semantic of set_pwr_src
set_pwr_src is now part of apm handlers group
in iwlcore

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>