KVM: arm/arm64: vgic_init: Cleanup reference to process_maintenance

One comment still mentioned process_maintenance operations after
commit af061499

 ("KVM: arm/arm64: vgic: Get rid of unnecessary
process_maintenance operation")

Update the comment to point to vgic_fold_lr_state instead, which
is where maintenance interrupts are taken care of.

Acked-by: Christoffer Dall <christoffer.dall@arm.com>
Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>