MFC r362294,r362647: hdac_intr_handler: keep working until global interrupt status clears

It is plausible that the hardware interrupts a host only when GIS goes
from zero to one. GIS is formed by OR-ing multiple hardware statuses,
so it's possible that a previously cleared status gets set again while
another status has not been cleared yet. Thus, there will be no new
interrupt as GIS always stayed set. If we don't re-examine GIS then we
can leave it set and never get another interrupt again.

Without this change I frequently saw a problem where snd_hda would stop
working. Setting dev.hdac.1.polling=1 would bring it back to life and
afterwards I could set polling back to zero. Sometimes the problem
started right after a boot, sometimes it happened after resuming from
S3, frequently it would occur when sound output and input are active
concurrently (such as during conferencing). I looked at HDAC_INTSTS
while the sound was not working and I saw that both HDAC_INTSTS_GIS and
HDAC_INTSTS_CIS were set, but there were no interrupts.