amdiommu: Fix device table segment base register offsets

Segment base registers are at 8-byte intervals, while the register
write helper takes a byte-aligned offset. This fixes
DEV_TAB_HARDWARE_ERROR events and associated peripheral I/O failures
on an Epyc-based system with 8-segment device tables.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D47752