staging: mt7621-mmc: Refactor dma setup process

Current code uses two functions for dma setup, msdc_dma_config and
msdc_dma_setup. By now msdc_dma_setup is nearly empty and mainly calls
msdc_dma_config, so the later one can be inline into the first
one. While doing this there is also some refactoring done.

Signed-off-by: Christian Lütke-Stetzkamp <christian@lkamp.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>