ethtool: move string arrays into common file

Introduce file net/ethtool/common.c for code shared by ioctl and netlink
ethtool interface. Move name tables of features, RSS hash functions,
tunables and PHY tunables into this file.

Signed-off-by: Michal Kubecek <mkubecek@suse.cz>
Reviewed-by: Jiri Pirko <jiri@mellanox.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>