greybus: gb-audio: Ensure i2c adapter struct exists before using

The current audio code uses i2c_get_adapter() without checking
that a non-NULL pointer is returned (i.e., that the i2c device
actually exists).  When that happens, the system panics.
Fix the potential panic by erroring out with -ENODEV when
i2c_get_adapter() returns NULL.

CC: John Stultz <john.stultz@linaro.com>
Signed-off-by: Mark Greer <mgreer@animalcreek.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>