Bluetooth: 6lowpan: Check transmit errors for multicast packets

We did not return error if multicast packet transmit failed.
This might not be desired so return error also in this case.
If there are multiple 6lowpan devices where the multicast packet
is sent, then return error even if sending to only one of them fails.

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>