qlcnic: add loopback diagnostic test

Loopback test (offline) added in ethtool self test.
o Set device in loopback mode
o Send packets
o Process receive packets in qlcnic_process_rcv_ring_diag()
o Compare packets
o Reset device in normal mode.

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