ALSA: hda_intel: fix handling of non-completion stream interrupts

Check that the interrupt raised for a stream is actually a buffer
completion interrupt before handling it as one.  Otherwise, memory
errors or FIFO xruns would be interpreted as a pointer update and could
break the stream timing.

Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>