bnx2x: Stop management traffic in loopback test

Since management traffic cannot go on the wire while configured to loopback,
simply stop it to avoid race condition when the MAC is set to loopback in the
middle of a packet

Signed-off-by: Eilon Greenstein <eilong@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>