qlcnic: Enhance diagnostic loopback error codes.

o Enhanced the driver to use standard Linux error codes
o Return a unique error code to indicate loopback is in progress

Signed-off-by: Jitendra Kalsaria <jitendra.kalsaria@qlogic.com>
Signed-off-by: Himanshu Madhani <himanshu.madhani@qlogic.com>
Signed-off-by: David S. Miller <davem@davemloft.net>