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