qlcnic: Stop traffic before performing loopback test

Before conducting loopback test by sending packets, driver should stop transmit
queue and turn off carrier.

Signed-off-by: Jitendra Kalsaria <jitendra.kalsaria@qlogic.com>
Signed-off-by: Shahed Shaikh <shahed.shaikh@qlogic.com>
Signed-off-by: David S. Miller <davem@davemloft.net>