[media] v4l: Load I2C modules based on modalias

[media] v4l: Load I2C modules based on modalias

When creating a new sub-device, The V4L I2C subdev API has historically
required drivers to pass the name of the module that implements support
for the I2C device.

I2C modules can be loaded based on modaliases instead of the module
name. As the I2C device type name is already available to the
v4l2_i2c_new_subdev* functions, make the module name argument optional
and create a modalias based on the type name when no module name is
provided.

All in-tree drivers call those functions with a non-NULL module name
argument, this change is thus harmless.

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