mac80211: Remove superfluous if clause

ieee80211_rx_h_check returned RX_DROP_MONITOR in case the if statement
in question was true but the same return value is also used directly
after the if clause. Hence, we can just drop the whole if clause and as
such simplify the code.

Signed-off-by: Helmut Schaa <helmut.schaa@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>