mcast: net_device dev not used

ip6_mc_source(), ip6_mc_msfilter() as well as ip6_mc_msfget() declare
and assign dev but do not use the variable afterwards.

Signed-off-by: Hagen Paul Pfeifer <hagen@jauu.net>
Signed-off-by: David S. Miller <davem@davemloft.net>