weird fxp / timecounter interaction in top-of-tree

[Dag-ErlingSmørgrav]

With a kernel built from sources only a few hours old (in the hopes of
figuring out whether I could use the TSC in SMP mode), I hit a number
of strange bugs in the fxp and / or miibus code:

 - trying to unload fxp after the interface has been configured with
   dhclient causes a panic near the top of fxp_txeof (of course,
   somebody has broken dumps again...)

 - loading the if_fxp module (and miibus as a dependency) switches the
   timecounter hardware to PIIX.

 - switching the timecounter hardware manually to PIIX when if_fxp and
   miibus are loaded causes the machine to lock up.

DES
-- 
Dag-Erling Smørgrav - des at ofug.org