media: imx7-media-csi: Use functions instead of macros

Currently there is a macro for reading and another macro for writing
to the CSI registers.

Functions can do parameter type checking, which leads to a safer code,
so switch from macro to function implementation.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Reviewed-by: Rui Miguel Silva <rmfrfs@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>