gpio: 104-idi-48: Clear pending interrupt once in IRQ handler

Performing a read operation on the IRQ Status register will clear the
IRQ latch. Since a read operation on the IRQ Status register must be
performed in the IRQ handler in order to determine if the IRQ was in
fact generated by the device, the IRQ latch is consequently cleared by
the IRQ handler. A spinlock is used to guarantee that each IRQ is
serviced in the order it was received.

Signed-off-by: William Breathitt Gray <vilhelm.gray@gmail.com>