vlan: Pass ethtool get_ts_info queries to real device.

Commit a6111d3c "vlan: Pass SIOC[SG]HWTSTAMP ioctls to real device"
intended to enable hardware time stamping on VLAN interfaces, but
passing SIOCSHWTSTAMP is only half of the story. This patch adds
the second half, by letting user space find out the time stamping
capabilities of the device backing a VLAN interface.

Signed-off-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>