ASoC: fsl_spdif: fix sysclk_df type

According to RM SPDIF STC SYSCLK_DF field is 9-bit wide, values
being in 0..511 range. Use a proper type to handle sysclk_df.

Signed-off-by: Viorel Suman <viorel.suman@nxp.com>
Signed-off-by: Mark Brown <broonie@kernel.org>