[MMC] MMCI: Ensure that we read all data from FIFO.

[MMC] MMCI: Ensure that we read all data from FIFO.

When we are nearing the end of a transfer from the card, we may not
have sufficient data in the FIFO to trigger a "half FIFO full" IRQ.
Therefore, when we have less than MCI_FIFOSIZE bytes left, switch to
"FIFO contains data" IRQ mode instead.