ASoC: Fix build dependency for SND_ATMEL_SOC_SSC

Make SND_ATMEL_SOC_SSC select ATMEL_SSC to fix below build errors:

  LD      .tmp_vmlinux1
sound/built-in.o: In function `atmel_ssc_remove':
sound/soc/atmel/atmel_ssc_dai.c:713: undefined reference to `ssc_free'
sound/built-in.o: In function `atmel_ssc_probe':
sound/soc/atmel/atmel_ssc_dai.c:700: undefined reference to `ssc_request'
sound/built-in.o: In function `atmel_ssc_set_audio':
sound/soc/atmel/atmel_ssc_dai.c:845: undefined reference to `ssc_request'
sound/soc/atmel/atmel_ssc_dai.c:851: undefined reference to `ssc_free'
make: *** [.tmp_vmlinux1] Error 1

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>