staging: mt7621-mmc: Remove unused field burstsz from msdc_dma

The burstsz field in msdc_dma is set to a constant value and later
only used two times, using the constant directly in that places allows
to remove the field and improves readability.

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