staging: mt7621-mmc: Start cleanup of msdc_dma_config

Currently the msdc_dma_config function has some variables, that are
not needed, uses the macro msdc_init_bd that is only used here and
does not use the for_each_sg iterator. That last fact could cause a
bug if the scatterlist is chained.

The function is changed to remove these things, but none of the
changes should change the behavior of it (despite the case of a linked
scatterlist).

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