V4L/DVB (10088): video: sh_mobile_ceu cleanups and comments

This patch cleans up the sh_mobile_ceu driver and adds comments and
constants to clarify the magic sequence in sh_mobile_ceu_capture().

Signed-off-by: Magnus Damm <damm@igel.co.jp>
Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>