[media] V4L: soc-camera: a missing mediabus code -> fourcc translation is not critical

soc_mbus_get_fmtdesc() returning NULL means only, that no standard
mediabus code -> fourcc conversion is known, this shouldn't be treated
as an error by drivers.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>