Bluetooth: add support for SIOCETHTOOL ETHTOOL_GET_TS_INFO

Bluetooth needs some way for user to get supported so_timestamping flags
for the different socket types.

Use SIOCETHTOOL API for this purpose. As hci_dev is not associated with
struct net_device, the existing implementation can't be reused, so we
add a small one here.

Add support (only) for ETHTOOL_GET_TS_INFO command. The API differs
slightly from netdev in that the result depends also on socket type.

Signed-off-by: Pauli Virtanen <pav@iki.fi>
Acked-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>