AT91 MMC 4 : Interrupt handler cleanup

This patch simplifies the AT91RM9200 MMC interrupt handler code so that
it doesn't re-read the Interrupt Status and Interrupt Mask registers
multiple times.

Also defined AT91_MCI_ERRORS instead of using the hard-coded 0xffff0000.

Signed-off-by: Andrew Victor <andrew@sanpeople.com>
Signed-off-by: Pierre Ossman <drzeus@drzeus.cx>