ASoC: atmel_ssc_dai: fix start event for I2S mode

According to the I2S specification information as following:
  - WS = 0, channel 1 (left)
  - WS = 1, channel 2 (right)
So, the start event should be TF/RF falling 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>
Cc: stable@vger.kernel.org