be2net: use -ENETDOWN error status when interface is down

Updating VF's tx-rate and FW-download are not allowed when the interface is
down. In such cases return -ENETDOWN to the stack.
Signed-off-by: Kalesh AP <kalesh.purayil@emulex.com>
Signed-off-by: Sathya Perla <sathya.perla@emulex.com>
Signed-off-by: David S. Miller <davem@davemloft.net>