pci: Only add special VF handling for direct children in bus methods

For activate/deactivate resource, use a more standard check at the
start of the function since the addition of the PCI_IOV code made this
more complex. For the three recently added methods, just add the
typical check at the beginning that I missed.

This wasn't always fatal as if your system only had PCI device_t's as
children of PCI bus devices it would happen to work ok, but if you
have a non-PCI child device (e.g. an ATA channel) then dereferencing
ivars for non-direct-children could fault.

Reported by: Cirrus-CI (via emaste)
Reviewed by: emaste
Fixes: 871b33ad65ba pci: Consistently use pci_vf_* for suballocated VF memory resources
Differential Revision: https://reviews.freebsd.org/D45499