mmc: mvsdio: workaround for spurious irqs

SDIO controllers found on Marvell Kirkwood SoCs seem to cause a late,
spurious irq although all interrupts have been disabled. This irq
doesn't do any harm, neither to HW nor driver. To avoid some
"unexpected irq" warning later, we workaround above issue by bailing
out of irq handler early, if we didn't expect any.

Signed-off-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Acked-by: Jason Cooper <jason@lakedaemon.net>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Chris Ball <chris@printf.net>