ethtool: add CHANNELS_NTF notification

Send ETHTOOL_MSG_CHANNELS_NTF notification whenever channel counts of
a network device are modified using ETHTOOL_MSG_CHANNELS_SET netlink
message or ETHTOOL_SCHANNELS ioctl request.

Signed-off-by: Michal Kubecek <mkubecek@suse.cz>
Signed-off-by: David S. Miller <davem@davemloft.net>