ASoC: rt5645: reset RT5645_AD_DA_MIXER at probe

RT5645_AD_DA_MIXER (0x29) register will not be reset to default after
SW reset. So we have to write it to its default value in i2c_probe.

Signed-off-by: Bard Liao <bardliao@realtek.com>
Signed-off-by: Mark Brown <broonie@kernel.org>