tests: net: mld: Fix ICMPv6 checksum calculation

The ICMP checksum was not placed to correct position in the
created test packet. This caused the checksum to be bad, and
because we now check the ICMP checksum, the packet was rejected
and the mld test failed.

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>