powerpc/4xx/uic: clear pending interrupt after irq type/pol change

When testing out gpio-keys with a button, a spurious
interrupt (and therefore a key press or release event)
gets triggered as soon as the driver enables the irq
line for the first time.

This patch clears any potential bogus generated interrupt
that was caused by the switching of the associated irq's
type and polarity.

Signed-off-by: Christian Lamparter <chunkeey@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>