[media] media: vb2: reset queued_count value during queue reinitialization

queued_count variable was left untouched during the queue reinitialization
in __vb2_queue_cancel, what might lead to mismatch between the real number
of queued buffers and queued_count variable.

Reported-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
CC: Pawel Osciak <pawel@osciak.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>