staging: mt7621-mmc: Refactor msdc_init_gpd_bd

The msdc_init_gpd_bd function is currently hard to read, because of
old, commented out code and a while loop, where a for loop is much
easier to read. Refactor it to make the code more readable.

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