ASoC: fsl_ssi: Rename i2smode to i2s_net

Since this i2smode also includes the setting of Network mode, it
should have it in the name. This patch also adds its MASK define.

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