net: ethtool: cabletest: Make ethnl_act_cable_test_tdr_cfg static

kbuild test robot is reporting:
net/ethtool/cabletest.c:230:5: warning: no previous prototype for

Mark the function as static.
Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ethtool/cabletest.c

@@ -227,7 +227,7 @@ cable_test_tdr_act_policy[ETHTOOL_A_CABLE_TEST_TDR_MAX + 1] = {
 };
 
 /* CABLE_TEST_TDR_ACT */
-int ethnl_act_cable_test_tdr_cfg(const struct nlattr *nest,
+static int ethnl_act_cable_test_tdr_cfg(const struct nlattr *nest,
 					struct genl_info *info,
 					struct phy_tdr_config *cfg)
 {