kni: fix multicast ioctl handling

kni: fix multicast ioctl handling

We did some (very basic) tests with IGMP, which involves adding
multicast addresses to ETH interfaces. This is done via the ip tool,
an example can be found on e.g.,

  http://superuser.com/questions/324824/linux-built-in-or-open-source-program-to-join-multicast-group

and this will fail on KNI interfaces because of an unimplemented ioctl
SIOCADDMULTI. The patch simply adds an empty callback for set_rx_mode
(typically used for setting up hardware) so that the ioctl succeeds.
This is the same thing as the Linux tap interface does.

Signed-off-by: Simon Kagstrom <simon.kagstrom@netinsight.net>
Signed-off-by: Johan Faltstrom <johan.faltstrom@netinsight.net>
Reviewed-by: Stephen Hemminger <stephen@networkplumber.org>
Acked-by: Helin Zhang <helin.zhang@intel.com>