net: icmpv6: Remove unused variable 'len' from input calls

Variable len is not used in net_icmpv6_input() and if required
it can be obtained from 'buf'.

Change-Id: I4b0710e1cf16cff9837173ad9d6908ec54ebafae
Signed-off-by: Ravi kumar Veeramally <ravikumar.veeramally@linux.intel.com>