LinuxKPI: pci: implementation of [lkpi_]pci_get_slot()

Like lkpi_pci_get_domain_bus_and_slot() implement lkpi_pci_get_slot()
using pci_find_bsf() instead of pci_find_dbsf() (no domain).
This is needed for a wireless driver.

Unfortunately the name [pci_get_slot()] collides with the native PCI
function. Add a guard around it and disable the use when the native
version is required (in lkpifill_pci_dev() and in bnxt/bnxt_en; if the
latter gets fixed we can probably also fix work around it in the former;
further conflicts in drm-kmod 6.1-lts, 6.6-lts, and master were resolved).

Sponsored by: The FreeBSD Foundation (initially)
MFC aftre: 3 days
Reviewed by: dumbbell
Differential Revision: https://reviews.freebsd.org/D52065