commit a43bd7e1 upstream.

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>
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>