[media] coda: fix error return value if v4l2_m2m_ctx_init fails

Use ret from the outer scope, instead of redefining it in the
conditional clause. That way the error value reaches the end of
the function as intended.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>