media: i2c: imx208: Use const 'struct bin_attribute' callback

The sysfs core now provides callback variants that explicitly take a
const pointer. Make use of it to match the attribute definition.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>