[media] soc-camera: Fix bytes per line computation for planar formats

The V4L2 specification defines bytesperline for planar formats as the
number of bytes per line for the largest plane. Modify
soc_mbus_bytes_per_line() accordingly.

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