irqchip/irq-pic32-evic: Fix bug with external interrupts.

The wrong external interrupt bits are being set, offset by 1.

Signed-off-by: Joshua Henderson <digitalpeer@digitalpeer.com>
Signed-off-by: Purna Chandra Mandal <purna.mandal@microchip.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>