media: imx7-media-csi: Use u32 for storing register reads

The CSI registers are 32-bit, so using u32 type is more suitable
for storing the values from register reads.

Switch from 'unsigned long' to 'u32' type.

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