V4L/DVB (13190): em28xx: fix panic that can occur when starting audio streaming

Because the counters were not reset when starting up streaming, they would
be reused from the previous run.  This can result in cases such that when the
second instance of streaming starts up, the "cnt" variable in
em28xx_audio_isocirq() can end up being negative, resulting in attempting to
write to memory before the start of runtime->dma_area (as well as having a
negative number of bytes to copy).

Signed-off-by: Devin Heitmueller <dheitmueller@kernellabs.com>
CC: stable@kernel.org
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>