Detect invalid PCI devices more correctly in PCI interrupt router drivers.

Detect invalid PCI devices more correctly in PCI interrupt router drivers.

- Check for an invalid device (vendor is invalid) before reading the
  header type register when examining function 0 of a possible device.
- When iterating over functions of a device, reject any device whose
  16-bit vendor is invalid rather than requiring the full 32-bit
  vendor+device to be all 1's.  In practice the latter check is
  probably fine, but checking the vendor is what the PCI spec
  recommends.

Reviewed by:	imp
MFC after:	2 weeks
Differential Revision:	https://reviews.freebsd.org/D21147