media: nxp: imx8-isi: Remove unused offset in mxc_isi_reg and use BIT() macro for mask

Preserve clarity by removing the unused 'offset' field in struct
mxc_isi_reg, as it duplicates information already indicated by the mask
and remains unused.

Improve readability by replacing hex value masks with the BIT() macro.

No functional change.

Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Frank Li <Frank.Li@nxp.com>
Link: https://lore.kernel.org/r/20250522-8qxp_camera-v5-3-d4be869fdb7e@nxp.com


Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>