arm/gic: fix a KASSERT in arm_gic_reserve_msi_range()

Fix the off-by-one in the KASSERT in arm_gic_reserve_msi_range()
allowing the MSIs being allocated at the end of the IRQ range.

Reported by: bz
Submitted by: andrew
Reviewed by: andrew
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D36291