[SCSI] cxgb3i: check for setup netdev

If the netdev has not been setup when the host is, we will oops when
the iscsi layer calls into the driver and a it tries to reference the
netdev in hba->ndev.

This can happen if the iscsi driver is loaded before ifup is
done. This patch just adds a check, so we can gracefully fail the
operation.

Signed-off-by: Mike Christie <michaelc@cs.wisc.edu>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>