ArmPkg/Gic: Fix boundary checking bug

mGicNumInterrupts is the total number of interrupts, so the interrupt
ID equal to mGicNumInterrupts is also invalid.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Heyi Guo <heyi.guo@linaro.org>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>

ArmPkg/Drivers/ArmGic/ArmGicCommonDxe.c

@@ -62,7 +62,7 @@ RegisterInterruptSource (
   IN HARDWARE_INTERRUPT_HANDLER         Handler
   )
 {
-  if (Source > mGicNumInterrupts) {
+  if (Source >= mGicNumInterrupts) {
     ASSERT(FALSE);
     return EFI_UNSUPPORTED;
   }