bcm2835_sdhci: roll back r354823

r354823 kicked DATA_END handling out of the DMA interrupt path "to make
things easy", but this was likely a mistake -- if we know we're done after
we've finished pending DMA operations, we should go ahead and acknowledge
it rather than waiting for the controller to finalize it. If it's not ready,
we'll simply re-enable interrupts and wait for it anyways, to be re-entered
in sdhci_data_intr.