media: vicodec: check for valid format in v4l2_fwht_en/decode

These functions did not return an error if state->info was NULL
or an unsupported pixelformat was selected (should not happen,
but just to be on the safe side).

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>