Bluetooth: Stop ack_timer if ERTM enters in Local Busy or SREJ_SENT

The ack_timer is implemation specific, disabling it in such situation
avoids some potencial errors in the ERTM protocol.

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>