ethtool: set message mask with DEBUG_SET request

Implement DEBUG_SET netlink request to set debugging settings for a device.
At the moment, only message mask corresponding to message level as set by
ETHTOOL_SMSGLVL ioctl request can be set. (It is called message level in
ioctl interface but almost all drivers interpret it as a bit mask.)

Signed-off-by: Michal Kubecek <mkubecek@suse.cz>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>