Add a boolean and an int a driver can set in their 'struct pci_driver'
definiotion to raise or lower the default LinuxKPI pci return value.
This is helpful in case of multiple drivers with overlapping IDs,
such as iwlwifi(4) and iwm(4).
The reason for the bool and the int is that 0 (BUS_PROBE_SPECIFIC)
is a valid return but would also be the unintialised (bss) value.
Having the bool allows us to check if we should apply the specific
return value or stay with the default. Given 'struct pci_driver'
is usually statically initialised we cannot set an easy global default
to always use.
Suggested by: jhb
Sponsored by: The FreeBSD Foundation
MFC after: 3 days
X-MFC: struct members need to go to the end for stable/13
Differential Revision: https://reviews.freebsd.org/D33915