mcastv6: Local variable shadows function argument

The local variable 'idev' shadows the function argument 'idev' to
ip6_mc_add_src(). Fixed by removing the local declaration, as pmc->idev
should be identical with 'idev' passed as argument.

Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
Acked-by: David L Stevens <dlstevens@us.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>