ASoC: rsnd: make sure variable name for 44.1kHz/48kHz

This driver is assuming that
RBGA is used as source clock of 44.1kHz category, and
RBGB is used as source clock of 48kHz category.
This patch clarifies the variable name.

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