ASoC: rsnd: tidyup register naming of BUSIF_MODE

Gen1:SRU has only 1 BUSIF_MODE,
but Gen2:SSIU/SCU has SRCm_BUSIF_MODE, and SSIn_BUSIF_MODE.
This patch rename current BUSIF_MODE to SRC_BUSIF_MODE.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Mark Brown <broonie@linaro.org>