mac80211: split ieee80211_drop_unencrypted

Currently, ieee80211_drop_unencrypted is called
from management and data frame context, and the
different contexts pass different frames. This
could lead to it processing an 802.3 frame as an
802.11 frame when MFP is enabled.

Move the MFP part of ieee80211_drop_unencrypted
into a new function that is only called for mgmt
frames.

Cc: Jouni Malinen <j@w1.fi>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>