Don't return -10000 as the probe priority. That's lower than what
BUS_PROBE_HOOVER is. Drivers like proto(4), when compiled into the
kernel or preloaded, will render your system useless by virtue of
attaching to your PCI busses.

Return BUS_PROBE_GENERIC instead. It's just the next priority up
from BUS_PROBE_HOOVER. No other meaning has been give to its use.
While BUS_PROBE_DEFAULT seems like a better candidate, it's hard
not to think that there must be some reason why these drivers
return -10000 in the first place.

Differential Revision: D2705