net: icmpv6: Print received packet type as string

This helps debugging as printable string of the ICMPv6 type is
printed when packet is received if debugging is active.

Change-Id: I22b84bb6b28db7fba030699af3e561a0775b53d2
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>