pci: Narrow the scope of recently-added PCI_IOV #ifdefs

Push the #ifdefs down into the function body instead of defining
functions conditionally for ease of readability. These aren't
critical paths, so one extra branch in the !PCI_IOV case is not a big
deal.

Requested by: jrtc27
Differential Revision: https://reviews.freebsd.org/D45877