iommu_gas: Correct a broken KASSERT

If iommu_gas_find_space() ever called iommu_gas_uppermatch(), and it
succeeded in allocating space, then the subsequent KASSERT would be
triggered. Change that KASSERT to accept either success or ENOMEM.

MFC after: 1 week

(cherry picked from commit e6775534aee1963a39e5ee762b8eab1d7dfb1b6b)