LinuxKPI: return value for linux_pci_probe()

In linux_pci_probe() return BUS_PROBE_DEFAULT rather than 0
(which actually is BUS_PROBE_SPECIFIC and means 'Only this
driver can use this device').

Sponsored by: The FreeBSD Foundation
MFC after: 3 days
Reviewed by: hselasky
Differential Revision: https://reviews.freebsd.org/D33913