media: v4l: subdev: Add a function to set an I²C sub-device's name

v4l2_i2c_subdev_set_name() can be used to assign a name to a sub-device.
This way uniform names can be formed easily without having to resort to
things such as snprintf in drivers.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>