Bluetooth: Split l2cap_data_channel_sframe()

Create a function for each type fo S-frame and avoid a lot of nested
code.

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>