ASoC: sam9x5_wm8731: change to work in DSP A mode

Change sam9x5 with wm8731 work in DSP A mode, this will fix the
left/right channel swap issue.

Signed-off-by: Bo Shen <voice.shen@atmel.com>
Tested-by: Richard Genoud <richard.genoud@gmail.com>
Signed-off-by: Mark Brown <broonie@linaro.org>