net: ethtool: self_test: Mark interface in testing operative status

When an interface is executing a self test, put the interface into
operative status testing.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>