ath9k: fix a WEP crypto related regression

commit b4a82a0 "ath9k_hw: fix interpretation of the rx KeyMiss flag"
fixed the interpretation of the KeyMiss flag for keycache based lookups,
however WEP encryption uses a static index, so KeyMiss is always asserted
for it, even though frames are decrypted properly.
Fix this by clearing the ATH9K_RXERR_KEYMISS flag if no keycache based
lookup was performed.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Cc: stable@vger.kernel.org
Reported-by: Laurent Bonnans <bonnans.l@gmail.com>
Reported-by: Jurica Vukadin <u.ra604@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>