ASoC: atmel_ssc_dai: fix the setting for DSP mode

When SCC work in DSP A mode, the data outputs/inputs are shift out on
falling edge, the frame sync are sample on the rising edge.

Reported-by: Songjun Wu <songjun.wu@atmel.com>
Signed-off-by: Bo Shen <voice.shen@atmel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>