ASoC: rnsd: fix build regression without CONFIG_OF

The r-car sound driver only works when CONFIG_OF is set, and
after a recent change has a compile-time dependency as well:

sound/built-in.o: In function `rsnd_dma_request_channel':
:(.text+0x9fb84): undefined reference to `of_dma_request_slave_channel'

This could be fixed either by adding a static inline wrapper
for the function, or by adding a Kconfig dependency. This
implements the second approach, which seems appropriate
because the driver in fact has a hard dependency.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 72adc61f ("ASoC: rsnd: 1st DMAC dma-names cares subnode")
Signed-off-by: Mark Brown <broonie@kernel.org>