ath: Missed to clear key4 of micentry

key4 of micentry is used, if ATH_CRYPT_CAP_MIC_COMBINED is set.
But is not cleared on key cache reset.

Cc: stable@kernel.org
Signed-off-by: Rajkumar Manoharan <rmanoharan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>