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.

index 2b3030415573..07114b0b0dc1 100644
@@ -36,6 +36,7 @@ config SND_SOC_SH4_SIU
 
 config SND_SOC_RCAR
 	tristate "R-Car series SRU/SCU/SSIU/SSI support"
+	depends on DMA_OF
 	select SND_SIMPLE_CARD
 	select REGMAP_MMIO
 	help

Applied, thanks.