A race condition in the MII layer causes spurious events, see:
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=252165
. Add a lock to serialize operations on the MII bus and skip ticks while
it is held.
I have tested this patch with my Belkin USB 10/100 Ethernet Adapter
F5D5050.
Signed-off-by: Rafael Kitover <rkitover@gmail.com>