ASoC: fix SND_PXA2XX_LIB Kconfig warning

Fix following warning observed when SND_PXA2XX_SOC is set and SND_ARM isn't:

warning: (SND_PXA2XX_AC97 && SOUND && !M68K && SND && SND_ARM && ARCH_PXA ||
SND_PXA2XX_SOC && SOUND && !M68K && SND && SND_SOC && ARCH_PXA) selects
SND_PXA2XX_LIB which has unmet direct dependencies (SOUND && !M68K && SND &&
SND_ARM)

Signed-off-by: Dmitry Artamonow <mad_soft@inbox.ru>
Acked-by: Liam Girdwood <lrg@slimlogic.co.uk>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>