Bluetooth: Fix memory leak of S-frames into L2CAP

l2cap_data_channel do not free the S-frame, so we free it here.

Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>
Reviewed-by: João Paulo Rechi Vita <jprvita@profusion.mobi>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>