net: phy: Send notifier when starting the cable test

Given that it takes time to run a cable test, send a notify message at
the start, as well as when it is completed.

v3:
EMSGSIZE when ethnl_bcastmsg_put() fails
Print an error message on failure, since this is a void function.

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>