ARM: mmci: avoid unnecessary switch to data available PIO interrupts

We don't need to switch to data available interrupts if there's at
least half a FIFO depth worth of data remaining, as we'll still get
the FIFO half full interrupt.  Keep this interrupt masked off until
we have less than half the FIFO depth worth of data remaining.

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