cxgb4: Fix -Wunused-const-variable warning

If CONFIG_PCI_IOV is n, make W=1 warns:

drivers/net/ethernet/chelsio/cxgb4/cxgb4_main.c:3909:33:
 warning: ‘cxgb4_mgmt_ethtool_ops’ defined but not used [-Wunused-const-variable=]
 static const struct ethtool_ops cxgb4_mgmt_ethtool_ops = {
                                 ^~~~~~~~~~~~~~~~~~~~~~

Move it into #ifdef block to fix this.

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>