mac80211: move dynamic PS timeout to hardware config

This will be needed for drivers that set the
IEEE80211_HW_NO_STACK_DYNAMIC_PS flag and still
want to handle dynamic PS.

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