ASoC: fsl-ssi: Fix i2s_mode variable setup

In fsl_ssi_hw_params() we update the I2S and NET bits using the i2s_mode
variable. The fsl_ssi_set_dai_fmt() function only writes the i2s-mode to
the i2s_mode variable and not the NET bit. This fixes it by adding that
bit to i2s_mode.

Signed-off-by: Markus Pargmann <mpa@pengutronix.de>
Signed-off-by: Mark Brown <broonie@linaro.org>