ethtool: provide message mask with DEBUG_GET request

Implement DEBUG_GET request to get debugging settings for a device. At the
moment, only message mask corresponding to message level as reported by
ETHTOOL_GMSGLVL ioctl request is provided. (It is called message level in
ioctl interface but almost all drivers interpret it as a bit mask.)

As part of the implementation, provide symbolic names for message mask bits
as ETH_SS_MSG_CLASSES string set.

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