ASoC: tfa9879: Export OF device ID as module alias

The I2C core always reports a MODALIAS of the form i2c:<foo> even if the
device was registered via OF, this means that exporting the OF device ID
table device aliases in the module is not needed. But in order to change
how the core reports modaliases to user-space, it's better to export it.

Before this patch:

$ modinfo sound/soc/codecs/snd-soc-tfa9879.ko  | grep alias
alias:          i2c:tfa9879

After this patch:

$ modinfo sound/soc/codecs/snd-soc-tfa9879.ko  | grep alias
alias:          i2c:tfa9879
alias:          of:N*T*Cnxp,tfa9879C*
alias:          of:N*T*Cnxp,tfa9879

Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Signed-off-by: Mark Brown <broonie@kernel.org>