Add a lock for MII bus operations and drop ticks if the lock is held to
work-around a race condition in the MII bus layer, see:
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=252165
. Tested with the EVB-LAN7800LC-1 MICROCHIP LAN7800 evaluation board.
This should fix Ethernet on the Raspberry Pi 3 and some other models
possibly.
Signed-off-by: Rafael Kitover <rkitover@gmail.com>