dmaengine: Make __dma_request_slave_channel_compat() name argument constant

Inline function __dma_request_slave_channel_compat() doesn't modify "name"
argument but passes it to dma_request_slave_channel() which already takes
it as a constant.

Signed-off-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>