media: ov8856: Modify ov8856 register reading function to be simplified

We use put_unaligned_be16() to be simplified for setting register address
in ov8856_read_reg() and use sizeof() to be better suited for bytes
copying.

Signed-off-by: Ben Kao <ben.kao@intel.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>