bnxt_en: Add support for fw managed link down feature.

In the current code, the driver will not shutdown the link during
IFDOWN if there are still VFs sharing the port.  Newer firmware will
manage the link down decision when the port is shared by VFs, so
we can just call firmware to shutdown the port unconditionally and
let firmware make the final decision.

Reviewed-by: Edwin Peer <edwin.peer@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>