ASoC: atmel-classd: select correct Kconfig symbol

SND_ATMEL_SOC_CLASSD selects SND_ATMEL_SOC_DMA but the driver itself
handles its own DMA operations and doesn't need anything from
atmel-pcm-dma.c or atmel_ssc_dai.c.

Replace SND_ATMEL_SOC_DMA by SND_SOC_GENERIC_DMAENGINE_PCM which is the
only one actually required.

This may end up in a configuration leading to a link error:

sound/soc/atmel/atmel_ssc_dai.o: In function `atmel_ssc_set_audio':
atmel_ssc_dai.c:(.text+0x79c): undefined reference to `atmel_pcm_dma_platform_register'
atmel_ssc_dai.c:(.text+0x79c): relocation truncated to fit: R_AARCH64_CALL26 against undefined symbol `atmel_pcm_dma_platform_register'
sound/soc/atmel/atmel_ssc_dai.o: In function `atmel_ssc_put_audio':
atmel_ssc_dai.c:(.text+0xf24): undefined reference to `atmel_pcm_dma_platform_unregister'
atmel_ssc_dai.c:(.text+0xf24): relocation truncated to fit: R_AARCH64_CALL26 against undefined symbol `atmel_pcm_dma_platform_unregister'

Tested on sama5d2 xplained with the following configuration
where nothing selects SND_ATMEL_SOC_DMA:

CONFIG_SND_ATMEL_SOC=y
CONFIG_SND_ATMEL_SOC_CLASSD=y

Reported-by: Arnd Bergmann <arnd@arndb.de>
Tested-by: Arnd Bergmann <arnd@arndb.de>
Fixes: e0a25b6d

 ("ASoC: atmel-classd: add the Audio Class D Amplifier")
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Mark Brown <broonie@kernel.org>