gpio: zynq: Clear pending interrupt when enabling a IRQ

The Zynq GPIO controller does not disable the interrupt detection when the
interrupt is masked and only disables the propagation of the interrupt. This
means when the controller detects an interrupt condition while the interrupt is
logically disabled (and masked) it will propagate the recorded interrupt event
once the interrupt is enabled. This will cause the interrupt consumer to see
spurious interrupts to prevent this first make sure that the interrupt is not
asserted and then enable it.

E.g. when a interrupt is requested with request_irq() it will be configured
according to the requested type (edge/level triggered, etc.) after that it will
be enabled. But the detection circuit might have already registered a false
interrupt before the interrupt type was correctly configured and once the
interrupt is unmasked this false interrupt will be propagated and the interrupt
handler for the just request interrupt will called.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>