regmap: irq: clear status when disable irq

clear the status bit if the mask register doesn't prevent
the chip level irq from being asserted

OR in the following sequence, there will be irq storm happens:
1) interrupt is triggered;
2) another thread disables it(the mask bit is set);
3) _Then_ the interrupt thread is not ACKed(the status bit is not cleared),
   and it's ignored;
4) if the irq is still asserted because of the uncleared status bit,
   the irq storm happens;

Signed-off-by: Yi Zhang <yizhang@marvell.com>
Signed-off-by: Mark Brown <broonie@linaro.org>