bnx2x: Force link UP when the interface is in LOOPBACK mode

When the interface does not have carrier but when it's put into
loopback mode (for tests), it does not make sense to not have
the carrier. So force it!

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