be2net: clarify promiscuous cmd with a comment

The promiscous cmd config code gives an impression that
setting a port to promisc mode will unset the other port.
This is not the case and is clarified with a comment.

Signed-off-by: Sathya Perla <sathyap@serverengines.com>
Signed-off-by: David S. Miller <davem@davemloft.net>