ath9k_hw: small optimization in ar9002_hw_get_isr

ah->config.rx_intr_mitigation does not need to be checked before checking
the rx interrupt mask for AR_ISR_RXMINTR or AR_ISR_RXINTM, as those
interrupts will be masked out if rx interrupt mitigation is disabled.

Avoid reading AR_ISR_S5_S twice by reordering the code to be more concise.

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