bcm2835-mmc: Only claim one DMA channel

With both MMC controllers enabled there are few DMA channels left. The
bcm2835-mmc driver only uses DMA in one direction at a time, so it
doesn't need to claim two channels.

See: https://github.com/raspberrypi/linux/issues/1327



Signed-off-by: Phil Elwell <phil@raspberrypi.org>