media: v4l2: Fix v4l2_i2c_subdev_set_name function documentation

The doc says the I²C device's name is used if devname is NULL, but
actually the I²C device driver's name is used.

Fixes: 06582930

 ("media: v4l: subdev: Add a function to set an I²C sub-device's name")
Signed-off-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>