Bluetooth: 6lowpan: Don't drop packets when run out of credits

Since l2cap_chan_send will now queue the packets there is no point in
checking the credits anymore.

Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>
Acked-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>