[media] V4L: sh_mobile_ceu_camera: output image sizes must be a multiple of 4

CEU can only produce images with 4 pixel aligned width and height.
Enforce this alignment, adjust comments and simplify some calculations.

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