x86: Adjust base addr for PCI MCFG regions

Each bus gets 1 MB of address space; the actual base address for an
MCFG bus range is the address from the table plus the starting bus
number times 1 MB.

The PCI spec is unclear on this point, but this change matches what
Linux does, which is likely enough of a de facto standard regardless
of what any de jure standard might attempt to say.

Fixes: f54a3890b1f4 ("x86: Support multiple PCI MCFG regions")
Reviewed by: jhb
Differential Revision: https://reviews.freebsd.org/D43206

(cherry picked from commit 61e2161367bb4e68e603f53b1107f9427f988861)