mac80211: Fix drop_unencrypted for MFP with hwaccel

Commit bef5d1c7 split
ieee80211_drop_unencrypted() into separate functions that are used for
Data and Management frames. However, it did not handle the
RX_FLAG_DECRYPTED correctly for Management frames:
ieee80211_drop_unencrypted() can only return 0 for Management frames,
so there is no point in calling it here. Instead, just check the
status->flag directly.

Signed-off-by: Jouni Malinen <j@w1.fi>
Signed-off-by: John W. Linville <linville@tuxdriver.com>