net: openthread: Don't regsiter mcast addresses in Zephyr multiple times
net: openthread: Don't regsiter mcast addresses in Zephyr multiple times

OpenThread L2 could've called multicast address registration multiple
times for specific address, which resulted in having multiple entries
containing the same multicast IPv6 address in Zephyr.
Checking if address was already registered prevents that.

Signed-off-by: Robert Lubos <9d28997c6894a2d0321a7bb399169af7ce99b64e@nordicsemi.no>

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