ath9k_hw: clean up tx completion interrupt handling

TXQ_FLAG_TXOKINT_ENABLE and TXQ_FLAG_TXERRINT_ENABLE are always set and
used together, and they share the same bitmask in enum ath9k_tx_queue_flags.
Simplify the code that tests for these flags.

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