Bluetooth: Make l2cap_streaming_send() void.

It doesn't make sense to have a return value since we always set it
to 0.

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