ath9k_hw: check for asynchronous interrupts before bailing out

in ar9003_hw_get_isr we bail out if we don't have any primary
interrupts and synchronous interrupts, also make sure we don't
have any asynchronous interrupts

Signed-off-by: Mohammed Shafi Shajakhan <mohammed@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>