p54: fix queue stall due to underrun

Larry Finger discovered a weird behavior under load.
In essence, the queue's length count under runs,
which in turn renders the associated ac queue unusable.

Reported-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: Christian Lamparter <chunkeey@web.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>