netx-eth: initialize per device spinlock

The spinlock used in the netx-eth driver was never properly initialized.
This was noticed using CONFIG_DEBUG_SPINLOCK=y

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>