ath9k: fix intr_txqs setting

The struct ath_hw member intr_txqs is never reset/assigned outside
of ath9k_hw_init_queues() and with the used bitwise-or in the interrupt
handling ar9002_hw_get_isr() accumulates all ever set interrupt flags.

Fix this by using a pure assign instead of bitwise-or for the
first line (note: intr_txqs is only evaluated in case ATH9K_INT_TX bit
is set).

Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20211116220720.30145-1-ps.report@gmx.net