rocker: enable support for scattered packets

rocker supports the transmission of scattered packets, so let the kernel
know about it by setting the NETIF_F_SG bit in the device's features.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@resnulli.us>
Acked-by: Scott Feldman <sfeldma@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>