[media] s3c-camif: Fail on insufficient number of allocated buffers

Ensure the driver gets always at least its minimum required
number of buffers allocated by checking actual number of
allocated buffers in vb2_reqbufs(). And free any partially
allocated buffer queue with signaling an error to user space.
Without this patch applications may wait forever to dequeue
a filled buffer, because the hardware didn't even start after
VIDIOC_STREAMON, VIDIOC_QBUF calls, due to insufficient number
of empty buffers.

Reported-by: Alexander Nestorov <alexandernst@gmail.com>
Signed-off-by: Sylwester Nawrocki <sylvester.nawrocki@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>