bhyve: Rename a pci_cfgrw() parameter

pci_cfgrw() may be called via a write to the extended config space,
which is memory-mapped. In this case, the name "eax" is misleading.
Give it a more generic name. No functional change intended.

Reviewed by: corvink, jhb
MFC after: 1 week
Sponsored by: Innovate UK
Differential Revision: https://reviews.freebsd.org/D40732