bcm2835_sdhci: "fix" DMA on the RPi 4

According to the documentation I have, DREQ pacing should be required here.
The DREQ# hasn't changed since the BCM2835. As soon as we attempt to setup
DREQ, DMA stalls and there's no clear reason why as of yet. Setting this
back to NONE seems to work just as well, though it's yet to be determined if
this is a sustainable model in high-throughput scenarios.