ethtool: bring back missing comma in netdev features strings

The issue was introduced in commit eed2a12f.

Signed-off-by: Franco Fichtner <franco@lastsummer.de>
Acked-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Acked-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>