ASoC: fsl_ssi: Clean up set_dai_tdm_slot()

This patch replaces the register read with ssi->i2s_net for
simplification. It also removes masking SSIEN from scr value
since it's handled later by regmap_update_bits() to set this
scr value back.

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>