interrupt_controller: dw: use finalstatus instead of maskstatus

Instead of using maskstatus to see if an interrupt has fired,
use finalstatus instead. It has been observed that some
controllers do not update maskstatus correctly with incoming
interrupts, but finalstatus works fine.

FYI, the DW driver in Linux is also using finalstatus.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>