A small MMC driver cleanup.

Use the defined AT91_MCI_ERRORS in at91_mci_completed_command() instead
of specifying all the error bits individually.
Signed-off-by: Andrew Victor <andrew@sanpeople.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Pierre Ossman <drzeus@drzeus.cx>