mac80211: move WEP weak IV check

I suspect the compiler will do this optimisation
anyway, but it seems cleaner to move this into
the WEP switch case.

Also make rx_h_decrypt use a local variable for
the frame_control so that we don't need to reload
the hdr variable for this after linearizing.

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