net: ethernet: aquantia: Null pointer check for aq_nic_ndev_alloc.

We should check for a null pointer for aq_nic_ndev_alloc
instead netdev_priv.

Signed-off-by: Pavel Belous <pavel.belous@aquantia.com>
Reviewed-by: Lino Sanfilippo <LinoSanfilippo@gmx.de>
Signed-off-by: David S. Miller <davem@davemloft.net>