bnx2x: Correct advertised speed/duplex

If link is down due to management (and not due to actual phy link being lost),
driver should still behave as if the link is down; Querying via ethtool about
speed/duplex state should result in 'UNKNOWN' (same behaviour as when link is
actually down).

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