net: ethtool: netlink: Add support for triggering a cable test

Add new ethtool netlink calls to trigger the starting of a PHY cable
test.

Add Kconfig'ury to ETHTOOL_NETLINK so that PHYLIB is not a module when
ETHTOOL_NETLINK is builtin, which would result in kernel linking errors.

v2:
Remove unwanted white space change
Remove ethnl_cable_test_act_ops and use doit handler
Rename cable_test_set_policy cable_test_act_policy
Remove ETHTOOL_MSG_CABLE_TEST_ACT_REPLY

v3:
Remove ETHTOOL_MSG_CABLE_TEST_ACT_REPLY from documentation
Remove unused cable_test_get_policy
Add Reviewed-by tags

v4:
Remove unwanted blank line

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Michal Kubecek <mkubecek@suse.cz>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>