net: icmpv6: Echo-Reply seq and id fix

The sequence and identifier fields are 16-bit instead of 32-bit
long. This did not cause any issue in Echo-Reply but those two
fields should be set properly.

Change-Id: I5e4878f53d6bb37660d46d173159d27bbe0e94dc
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>