Bluetooth: Implement SendAck() Action on ERTM.

Shall be used to ack received frames, It must decide type of
acknowledgment between a RR frame, a RNR frame or transmission of
pending I-frames.
It also modifies l2cap_ertm_send() to report the number of frames sent.

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>