ASoC: fsl_ssi: remove duplicated flag setting in fsl_ssi_setup_reg_vals()

We don't need to set CCSR_SSI_SIER_RFF0_EN / CCSR_SSI_SIER_TFE0_EN bits
in reg->rx.sier / reg->tx.sier variables in a non-AC'97 mode considering we
had just initialized these variables to these very values unconditionally a
few lines earlier.

Signed-off-by: Maciej S. Szmigiero <mail@maciej.szmigiero.name>
Acked-by: Nicolin Chen <nicoleotsuka@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>