Fix handling of BUS_PROBE_NOWILDCARD in 'device_probe_child()'.

Device probe value of BUS_PROBE_NOWILDCARD should be treated specially only
if the device has a fixed devclass. Otherwise it should be interpreted just
as if the driver doesn't want to claim the device.

Prior to this change a device that was not claimed explicitly by its driver
would remain "attached" to the driver that returned BUS_PROBE_NOWILDCARD.
This would bump up the reference on 'driver->refs' and its 'dev->ops' would
point to the 'driver->ops'. When the driver is subsequently unloaded the
'dev->ops->cls' is left pointing to freed memory.

This fixes an easily reproducible #GP fault caused by loading and unloading
vmm.ko multiple times.

Differential Revision: https://reviews.freebsd.org/D2294
Reviewed by: imp, jhb
Discussed with: rstone
Reported by: Leon Dang (ldang@nahannisys.com)
MFC after: 2 weeks