ath9k_hw: Fix incorrect key_miss handling

Decryping frames on key_miss handling shouldn't be done for Michael
MIC failed frames as h/w would have already decrypted such frames
successfully anyway.

Also leaving CRC and PHY error(where the frame is going to be dropped
anyway), we are left to prcoess Decrypt error for which s/w decrypt is
selected anway and so having key_miss as a separate check doesn't serve
anything. So making key_miss handling mutually exlusive with other RX
status handling makes much more sense.

This patch addresses an issue with STA not reporting MIC failure events
resulting in STA being disconnected immediately.

Cc: stable@kernel.org
Signed-off-by: Senthil Balasubramanian <senthilb@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>