net: ipv6: Remove incorrect length assignment in MLDv2
net: ipv6: Remove incorrect length assignment in MLDv2

This assignment causes the later code to overwrite part of the
packet, making it malformed.

Signed-off-by: Björn Stenberg <8a2957e0063f2abf61bd145b6236a74a3708ac54@haxx.se>

Vudentz/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,Vudentz/zephyr,punitvara/zephyr,galak/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,explora26/zephyr,nashif/zephyr,explora26/zephyr,GiulianoFranchetto/zephyr,finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,ldts/zephyr,finikorg/zephyr,explora26/zephyr,nashif/zephyr,punitvara/zephyr,Vudentz/zephyr,Vudentz/zephyr,Vudentz/zephyr,ldts/zephyr,finikorg/zephyr,explora26/zephyr,zephyrproject-rtos/zephyr,ldts/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,ldts/zephyr,punitvara/zephyr,ldts/zephyr,galak/zephyr,explora26/zephyr,finikorg/zephyr,nashif/zephyr,punitvara/zephyr,GiulianoFranchetto/zephyr,punitvara/zephyr,finikorg/zephyr,GiulianoFranchetto/zephyr