[media] v4l: Remove hardcoded module names passed to v4l2_i2c_new_subdev*

With the v4l2_i2c_new_subdev* functions now supporting loading modules
based on modaliases, replace the hardcoded module name passed to those
functions by NULL.

All corresponding I2C modules have been checked, and all of them include
a module aliases table with names corresponding to what the drivers
modified here use.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>