[MMC] MMCI: Manipulate IRQ masks according to the data FSM state.

[MMC] MMCI: Manipulate IRQ masks according to the data FSM state.

Prevent IRQ race conditions between the two handlers and within the
primecell itself.  Unfortunately, the primecell does not always mask
the FIFO empty interrupt we reach the end of a transfer to the card.
Also, we may receive the "data end" interrupt prior to finishing a
transfer from the card, so mask this off until we've read the last
bytes from the FIFO.