Bluetooth: Fix possible missing I-Frame acknowledgement

Make l2cap_ertm_send return number of pending I-Frames transmitted
instead of all (pending + retransmitted) I-Frames transmitted.

As only pending I-Frames are considered as acknowledgement, this could
lead to situation when no ACK was sent in __l2cap_send_ack (if only
already transmitted I-Frames were retransmitted).

Signed-off-by: Szymon Janc <szymon.janc@tieto.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>
Acked-by: Gustavo F. Padovan <padovan@profusion.mobi>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>