[media] v4l2: vb2: one more fix for REQBUFS()

Return immediately if the target number of buffers is the same as
the current one and memory access type doesn't change.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>