Bluetooth: Change the way we set ERTM mode as mandatory

If the socket type is SOCK_STREAM we set Enhanced Retransmisson Mode or
Streaming Mode as mandatory. That means that we will close the channel
if the other side doesn't support or request the the mandatory mode.
Basic mode can't be set as mandatory.

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>