bnx2x: dropless flow control not always functional

Since commit 3deb8167 "bnx2x: Add a periodic task for link PHY events"
link state changes can be detected not only via the attention flow but also
from the periodic task.
If the link state will change in such a manner (i.e., via the periodic task),
dropless flow-control will not be configured.

This patch remedies the issue, adding the missing configuration to all required
flows.

Signed-off-by: Dmitry Kravkov <dmitry@broadcom.com>
Signed-off-by: Yuval Mintz <yuvalmin@broadcom.com>
Signed-off-by: Ariel Elior <ariele@broadcom.com>
Signed-off-by: Eilon Greenstein <eilong@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>