p54: more cryptographic accelerator fixes

If we let the firmware do the data encryption, we have to remove the ICV and
(M)MIC at the end of the frame before we can give it back to mac80211.
Or, these data frames have a few trailing bytes on cooked monitor interfaces.

Signed-off-by: Christian Lamparter <chunkeey@web.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>