[media] media: vb2: fix allocation failure check

__vb2_queue_alloc function returns the number of successfully allocated
buffers. There is no point in checking if the returned value is negative.
If this function returns 0, videobuf2 should just return -ENOMEM to
userspace, because no driver can work without memory buffers.

Reported-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
CC: Pawel Osciak <pawel@osciak.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>