cxgb4: add loopback ethtool self-test

In this test, loopback pkt is created and sent on default queue.
The packet goes until the Multi Port Switch (MPS) just before
the MAC and based on the specified channel number, it either
goes outside the wire on one of the physical ports or looped
back to Rx path by MPS. In this case, we're specifying loopback
channel, instead of physical ports, so the packet gets looped
back to Rx path, instead of getting transmitted on the wire.

v3:
- Modify commit message to include test details.
v2:
- Add only loopback self-test.

Signed-off-by: Vishal Kulkarni <vishal@chelsio.com>
Acked-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>