bnx2x: Fix XMAC loopback test

Change XMAC loopback type from CORE LOCAL to LINE LOCAL for the BCM578xx due to intermittent problem with the loopback with this configuration.

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