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>