[media] mem2mem_testdev: Fix incorrect location of v4l2_m2m_release()

v4l2_m2m_release() was placed after the return statement and outside
any of the goto labels and hence was not getting executed under the
error exit path. This patch moves it under the exit path label.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Acked-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>