mac80211: refactor dynamic power save check

Refactor dynamic power save checks to a function of it's own for better
readibility. No functional changes.

Signed-off-by: Kalle Valo <kalle.valo@nokia.com>
Reviewed-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>