LinuxKPI: USB change successful probe result to BUS_PROBE_DEFAULT

Rather than returning 0 (which is BUS_PROBE_SPECIFIC) return
BUS_PROBE_DEFAULT to give other drivers a possible chance too
to attach.

Reported by: jhb
Reviewed by: hselasky, jhb, imp
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D34317