[PATCH] Intruduce DMA_28BIT_MASK

This patch introduces the DMA_28BIT_MASK constant in dma-mapping.h
ALSA drivers using this mask are changed to use the new constant.

Signed-off-by: Tobias Klauser <tklauser@nuerscht.ch>
Acked-by: Takashi Iwai <tiwai@suse.de>
Acked-by: Jaroslav Kysela <perex@suse.cz>