ASoC: rsnd: route setting is needed only Gen1

Renesas sound has SRC (= Sampling Rate Converter),
but, the HW implementation depends on its generation.
It was part of SRU on Gen1, and SCU on Gen2.
This SCU needs DMA transfer to use it.
Current rsnd driver is using it as DMA transfer buffer
(= no rate convert), and Gen1 is only supported at this point.

This patch cleanup it with focusing about SRC and Gen2 part.

rsnd_scu_set_route() is needed only Gen1.

This patch renames it to rsnd_scu_set_route_if_gen1()
and it adds comment to rsnd_reg member
in order to clarify it is used for Gen1.

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