Bluetooth: Refactor l2cap_send_sframe

The new implementation uses struct l2cap_ctrl to set up the sframe
fields, and also reduces duplicate acks by canceling the ack timer
whenever an RR or RNR frame is sent.  sframe PDU generation is also
split in to a separate function to separate it from the logic related
to the connection state and sframe type.

Signed-off-by: Mat Martineau <mathewm@codeaurora.org>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>