dma: Make the 'mask' parameter of __dma_request_channel const

The 'mask' parameter is not modified in __dma_request_channel and really
shouldn't be. Make this explicit by making the parameter const.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>