net: ethtool: Refactor identical get_ts_info implementations.

The vlan, macvlan and the bonding drivers call their "real" device driver
in order to report the time stamping capabilities.  Provide a core
ethtool helper function to avoid copy/paste in the stack.

Signed-off-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: Kory Maincent <kory.maincent@bootlin.com>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Reviewed-by: Jay Vosburgh <jay.vosburgh@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>