mac80211: fix truncated packets in cooked monitor rx

Cooked monitor rx was recently changed to use ieee80211_add_rx_radiotap_header
instead of generating only limited radiotap information.
ieee80211_add_rx_radiotap_header assumes that FCS info is still present if
the hardware supports receiving it, however when cooked monitor rx packets
are processed, FCS info has already been stripped.
Fix this by adding an extra flag indicating FCS presence.

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