qlcnic: add external loopback support

o Add external loopback test in self test:
  - Send set external loopback mode request to fw.
     To quiscent other storage functions.
  - Perform test
  - Send unset loopback mode request to fw.

o Rename ilb to lb.
o Update driver version 5.0.20.

Signed-off-by: Sucheta Chakraborty <sucheta.chakraborty@qlogic.com>
Signed-off-by: Amit Kumar Salecha <amit.salecha@qlogic.com>
Signed-off-by: David S. Miller <davem@davemloft.net>