mac80211: Fix duplicate frames on cooked monitor

Cleaning the ieee80211_rx_data.flags field here is wrong, instead the
flags should be valid accross processing the frame on different
interfaces. Fix this by removing the incorrect flags=0 assignment.

Introduced in commit 554891e6
(mac80211: move packet flags into packet).

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