media: vb2: don't call __vb2_queue_cancel if vb2_start_streaming failed

vb2_start_streaming() already rolls back the buffers, so there is no
need to call __vb2_queue_cancel(). Especially since __vb2_queue_cancel()
does too much, such as zeroing the q->queued_count value, causing vb2
to think that no buffers have been queued.

It appears that this call to __vb2_queue_cancel() is a left-over from
before commit b3379c62.

Fixes: b3379c62 ('vb2: only call start_streaming if sufficient buffers are queued')

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Cc: <stable@vger.kernel.org>      # for v4.16 and up
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>