GIC: fix return value from retrigger

The generic irq layer expects a return value of 0 for failure to
retrigger an interrupt.

The MSM architecture doesn't have a retrigger mechanism and needs
to rely on the tasklet to rerun an interrupt.

Hence fix the code to return a 0 instead of -ENXIO when no retrigger
mechanism is present. This will make the genirq code to retrigger
the interrupt via tasklet.

Change-Id: I6ae86101af1e7a6d39a9b724047dee61c751df2c
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>