Fixes a problem when module probes before i2c module is available

The driver crashed while a NULL pointer returned by i2c_get_adapter()
has been used to access the i2c bus functions.
The headphone probing function hb_hp_probe() now returns -EPROBE_DEFER
in case the i2c module has not been loaded yet.

Signed-off-by: Joerg Schambacher <joerg@i2audio.com>