mac80211: Protect Deauthentication frame when using MFP

When management frame protection (IEEE 802.11w) is used,
Deauthentication frame needs to be protected when the pairwise key is
configured. mac80211 was removing the station entry (and its keys)
before actually sending out the Deauthentication frame. Fix this by
reordering the code to send the frame before the station entry gets
removed. This matches an earlier change that handled the Disassociation
frame processing, but missed Deauthentication frames.

Signed-off-by: Jouni Malinen <jouni.malinen@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>