[media] v4l2: vb2: simplify __vb2_queue_free function

__vb2_queue_free function doesn't really return anything useful. This patch
removes support for the return value to simplify the code.

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>