attempt to deal with situations where the hardware has processed more

attempt to deal with situations where the hardware has processed more
than one block of data since the interrupt handler was last run.

when azalia detects these situations, it will print a line to the
dmesg buffer like:

stream 1: swpos XXXX hwpos XXXX, adding intr

getting these messages in itself is not bad, but if audio breaks, I
need to see these messages.  this will be changed to a DPRINTF once
I get more feedback.

ok deraadt