i2c: core: report OF style module alias for devices registered via OF

The buses should honor the firmware interface used to register the device,
but the I2C core reports a MODALIAS of the form i2c:<device> even for I2C
devices registered via OF.

This means that user-space will never get an OF stype uevent MODALIAS even
when the drivers modules contain aliases exported from both the I2C and OF
device ID tables. For example, an Atmel maXTouch Touchscreen registered by
a DT node with compatible "atmel,maxtouch" has the following module alias:

$ cat /sys/class/i2c-adapter/i2c-8/8-004b/modalias
i2c:maxtouch

So udev won't be able to auto-load a module for an OF-only device driver.
Many OF-only drivers duplicate the OF device ID table entries in an I2C ID
table only has a workaround for how the I2C core reports the module alias.

This patch changes the I2C core to report an OF related MODALIAS uevent if
the device was registered via OF. So for the previous example, after this
patch, the reported MODALIAS for the Atmel maXTouch will be the following:

$ cat /sys/class/i2c-adapter/i2c-8/8-004b/modalias
of:NtrackpadT<NULL>Catmel,maxtouch

NOTE: This patch may break out-of-tree drivers that were relying on this
      behavior, and only had an I2C device ID table even when the device
      was registered via OF. There are no remaining drivers in mainline
      that do this, but out-of-tree drivers have to be fixed and define
      a proper OF device ID table to have module auto-loading working.

Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Tested-by: Dmitry Mastykin <mastichi@gmail.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>