DM9000: Add mutex to protect access

Add a mutex to serialise access to the chip functions from
entries such as the ethtool and the MII code. This should
reduce the amount of time the spinlock is held to protect
the address register.

Signed-off-by: Ben Dooks <ben-linux@fluff.org>
Signed-off-by: Jeff Garzik <jeff@garzik.org>