mfd: pm8xxx-irq: mask non requested interrupt

When a non requested interrupt triggers, the genirq ends up calling
mask or mask_ack callback. This callback is expected to mask that
interrupt so it doesn't trigger us further.

There is a bug in the pm8xxx-irq driver where config[] entry for an
unrequested irq remains empty. So the mask or the mask_ack callback
ends up masking 0th interrupt.

Fix the mask and mask_ack callback to check if config[] is empty, if
so fill it in right then and mask the irq.

Change-Id: I85d3bfff92c52a1c3eb8123a9c3757660e20fac6
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>