Restrict PCIe maxslots to 0, instead of PCI_SLOTMAX

Summary:
PCIe only permits 1 device on an endpoint, so some devices ignore the device
part of B:D:F probing. Although ARI likely fixes this, not all platforms
support ARI completely or correctly, so some devices end up showing up 32
times on the bus.

This was found during bringup of POWER9/Talos, and has been tested on POWER9
and POWER8 hardware.

Reviewed by: leitao
Differential Revision: https://reviews.freebsd.org/D15461