staging: mt7621-mmc: Always use linked DMA mode

Current code uses linked DMA mode only when there is more than one
entry in the scatterlist. But the overhead of the linked DMA is very
small, so the code is easier when only using linked DMA and this is
also the way, the upstream driver mtk-sd handles it.

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