ovpn: add basic ethtool support

Implement support for basic ethtool functionality.

Note that ovpn is a virtual device driver, therefore
various ethtool APIs are just not meaningful and thus
not implemented.

Signed-off-by: Antonio Quartulli <antonio@openvpn.net>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/20250415-b4-ovpn-v26-22-577f6097b964@openvpn.net


Reviewed-by: Sabrina Dubroca <sd@queasysnail.net>
Tested-by: Oleksandr Natalenko <oleksandr@natalenko.name>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>