staging: mt7621-mmc: Remove initialisation in msdc_dma_config

Current code initialises the variable sg at the beginning of the
msdc_dma_config function. This is not necessary, because the variable
is assigned by the for_each_sg macro later on.

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