net/mlx5e: IPoIB, Add ethtool support to get child time stamping parameters

Add support to get time stamping capabilities using ethtool for
child interface.
Usage example:
	ethtool -T CHILD-DEVNAME

This change reuses the functionality of parent devices and does not
introduce any new logic.

Signed-off-by: Feras Daoud <ferasda@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>