xen/arm: gic-vgic: Fix the assert condition in vgic_connect_hw_irq

xen/arm: gic-vgic: Fix the assert condition in vgic_connect_hw_irq

Currently, the assert condition in vgic_connect_hw_irq does not
correspond to the comment above and result to hit the assertion
on HW IRQ disconnection.

Fix the condition so it corresponds to the comment and allows IRQ
disconnection on debug builds.

Fixes: ec2a2f1 ("ARM: VGIC: factor out vgic_connect_hw_irq()")
Signed-off-by: Andrii Anisov <andrii_anisov@epam.com>
Suggested-by: Stefan Nuernberger <snu@amazon.de>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
[julieng: Reword the commit message]
Acked-by: Julien Grall <julien.grall@arm.com>
Release-acked-by: Juergen Gross <jgross@suse.com>