lib: utils/irqchip: plic: Fix the off-by-one error in context save/restore helpers

plic->num_src holds the number of interrupt sources without interrupt
source 0 but the interrupt enable register includes a bit for the
interrupt source 0 in the first word.

Fixes: 415ecf28

 ("lib: irqchip/plic: Add context save/restore helpers")
Signed-off-by: Bin Meng <bmeng@tinylab.org>
Reviewed-by: Anup Patel <anup@brainfault.org>
Reviewed-by: Samuel Holland <samuel@sholland.org>