DMA-API: provide a helper to set both DMA and coherent DMA masks

Provide a helper to set both the DMA and coherent DMA masks to the
same value - this avoids duplicated code in a number of drivers,
sometimes with buggy error handling, and also allows us identify
which drivers do things differently.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

diff --cc Documentation/DMA-API-HOWTO.txt
Simple merge

diff --cc Documentation/DMA-API.txt
Simple merge

diff --cc include/linux/dma-mapping.h
Simple merge