mac80211: do not send duplicate data frames to the cooked monitor interface

I can't think of a valid use case for this aside from debugging (which can
also be done with a real monitor interface), and dropping these frames saves
some precious CPU cycles.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>