V4L/DVB: v4l: soc_camera: fix bound checking of mbus_fmt[] index

When code <= V4L2_MBUS_FMT_FIXED soc_mbus_get_fmtdesc returns a pointer to
mbus_fmt[x], where x < 0. Fix this.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
CC: stable@kernel.org
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>