greybus: audio_codec: Don't be tricky with the driver model

With the recent changes by Johan to the driver model of the greybus
code, the audio_codec no longer should need to provide a "dummy" driver
when registering the codec.  In fact, having it there causes the greybus
core to crash when inserting the module.  Removing it all fixes the
crash.

Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>