net: add external loopback test in ethtool self test

External loopback test can be performed by application without any driver
support on normal Ethernet cards.
But on CNA devices, where multiple functions share same physical port.
Here internal loopback test and external loopback test can be initiated by
multiple functions at same time. To co exist all functions, firmware need
to regulate what test can be run by which function. So before performing external
loopback test, command need to send to firmware, which will quiescent other functions.

User may not want to run external loopback test always. As special cable need to be
connected for this test.
So adding explicit flag in ethtool self test, which will specify interface
to perform external loopback test.
 ETH_TEST_FL_EXTERNAL_LB: Application set to request external loopback test
 ETH_TEST_FL_EXTERNAL_LB_DONE: Driver ack if test performed

Signed-off-by: Amit Kumar Salecha <amit.salecha@qlogic.com>
Reviewed-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>