staging: mt7621-mmc: Remove unused global dma_size

The global variable dma_size was previously used to select DMA or
non-DMA transfer mode based on the size of the data that should be
transferred. This option was removed preivously, so the variable is not
used any more and can be removed.

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