Bluetooth: 6lowpan: Return EAGAIN error also for multicast packets

Make sure that we are able to return EAGAIN from l2cap_chan_send()
even for multicast packets. The error code was ignored unncessarily.

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