OMAP: GPIO: clear/restore level/edge detect settings on mask/unmask

If IRQ triggering is enabled, it can trigger a pending interrupt
even for masked interrupts.  Any pending GPIO interrupts can
prevent the powerdomain from hitting retention.

Problem found, reported and additional review and testing by Chunquiu
Wang.

Tested-by: Chunquiu Wang <cqwang@motorola.com>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>