mac80211: move some code into ieee80211_invoke_rx_handlers

There is some duplicated code that sits in front of each function
call to ieee80211_invoke_rx_handlers() that can very well be part
of that function if it gets slightly different arguments.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>