ASoC: fsl_ssi: Clear FIFO directly in fsl_ssi_config()

The FIFO clear helper function is just one line of code now.
So it could be cleaned up by removing it and calling regmap
directly.

Meanwhile, FIFO clear could be applied to all use cases, not
confined to AC97. So this patch also moves FIFO clear in the
trigger() to fsl_ssi_config() and removes the AC97 check.

Note that SOR register is safe from offline_config HW limit.

Signed-off-by: Nicolin Chen <nicoleotsuka@gmail.com>
Tested-by: Caleb Crome <caleb@crome.org>
Tested-by: Maciej S. Szmigiero <mail@maciej.szmigiero.name>
Reviewed-by: Maciej S. Szmigiero <mail@maciej.szmigiero.name>
Signed-off-by: Mark Brown <broonie@kernel.org>