[media] si2165: switch to regmap

This avoids some low-level operations.
It has the benefit that now register values van be read from /sys/kernel/debug/regmap
The maximum register value is just a guess - all higher addresses read as zero.

Signed-off-by: Matthias Schwarzott <zzam@gentoo.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>