[BNX2]: More 5706S link down workaround.

The previous patches to workaround the 5706S on an HP blade were not
sufficient.  The link state still does not change properly in some
cases.  This patch adds polling to make it completely reliable.

Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>