Bluetooth: Fix double acking I-Frames when sending pending I-Frames

Pending I-Frame(s) are considered as acknowledgement. To void double
acking (via I-Frame and later via RR) clear ack timer when sending
first pending I-Frame.

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