[MMC] mmci: allow small data transfers

If a data transfer is small (less than a FIFO size) we would
hang waiting for the data to be read due to the PIO interrupt
not occuring.  We allowed for this in our PIO interrupt handler,
but not when setting up a data transfer.

Apply the "fix" when setting up a data transfer as well.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>