be2net: disable RSS when number of RXQs is reduced to 1 via set-channels

When *only* the default RXQ is used, the RSS policy must be disabled so
that all IP and no-IP traffic is placed into the default RXQ. If not,
IP traffic is dropped.

Also, issue the RSS_CONFIG cmd only if FW advertises RSS capability for
the interface.
Signed-off-by: Vasundhara Volam <vasundhara.volam@emulex.com>
Signed-off-by: Sathya Perla <sathya.perla@emulex.com>
Signed-off-by: David S. Miller <davem@davemloft.net>