dma-mapping: update the old macro DMA_nBIT_MASK related documentations

Update the old macro DMA_nBIT_MASK related documentations

Signed-off-by: Yang Hongyang<yanghy@cn.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2 files changed