ath9k_hw: fix potential spurious tx error bit interpretation

According to documentation, AR_ExcessiveRetries, AR_Filtered and
AR_FIFOUnderrun are only valid if AR_FrmXmitOK is clear.

Not checking this might result in suboptimal FIFO settings, unnecessary
retransmissions, or other connectivity issues.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Cc: stable@kernel.org
Signed-off-by: John W. Linville <linville@tuxdriver.com>