ASoC: ak4642: Add default return value in ak4642_modinit

If ak4642 driver was compiled without I2C configs,
ak4642_modinit return value will become un-stable.
This patch modify this bug

Reported-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Kuninori Morimoto <morimoto.kuninori@renesas.com>
Acked-by: Liam Girdwood <lrg@slimlogic.co.uk>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>