net: ethtool: Add helpers for reporting test results

The PHY drivers can use these helpers for reporting the results. The
results get translated into netlink attributes which are added to the
pre-allocated skbuf.

v3:
Poison phydev->skb
Return -EMSGSIZE when ethnl_bcastmsg_put() fails
Return valid error code when nla_nest_start() fails
Use u8 for results
Actually put u32 length into message

v4:
s/ENOTSUPP/EOPNOTSUPP/g

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