ASoC: fsl_spdif: Fix unnecessary check in fsl_spdif_probe()

The function fsl_spdif_probe() is only called with an openfirmware
platform device. Therefore there is no need to check that the passed
in device is NULL.

Signed-off-by: Zhang Shengju <zhangshengju@cmss.chinamobile.com>
Signed-off-by: Tang Bin <tangbin@cmss.chinamobile.com>
Link: https://lore.kernel.org/r/20200826150918.16116-1-tangbin@cmss.chinamobile.com
Signed-off-by: Mark Brown <broonie@kernel.org>