net: l2: openthread: Fix error logs on adding already present address

Error checking of otIp6AddUnicastAddress() and
otIp6SubscribeMulticastAddress() was added recently, however it wasn't
taken into account that those APIs return an error on attempt to
register an IPv6 address that is already present on the OT interface.
Therefore, add more specific error checks, to return silently in case
address was already present.

As those two APIs are not very consistent, and otIp6AddUnicastAddress()
returns OT_ERROR_INVALID_ARGS in such cases, add an extra check if the
address is already present before attempting to register the address.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>