staging: hv: add explanation of no-op set_multicast_list

I was going to remove netvsc_set_multicast_list() so I think it is worth
adding a comment since it isn't immediately clear why you would want
this.

Signed-off-by: Brandon Philips <brandon@ifup.org>
Acked-by: Hank Janssen <hjanssen@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>