stmmac: priv->lock can be used uninitialized

To reproduce: if connman (http://connman.net/

) is started,
inserting the stmmac module triggers a "BUG: spinlock bad magic on CPU#0".

Registering the device in stmmac_probe() sends a notification to connman
which brings the interface up before the lock is initialized.

Signed-off-by: Vlad Lungu <vlad.lungu@windriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>