iwlagn: clean up PM code

The transport callbacks might as well be undefined
when CONFIG_PM_SLEEP is not set, so ifdef all of
it out and make everything available for PM_SLEEP
only.

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