ack_timer should be cleared when sending ACK to avoid acking I-frames
twice.

This commit introduces helper function (only send ack, not clearing
timer) which is used by l2cap_send_ack and l2cap_ack_timeout. This is
to avoid clearing ack timer in timer function.
Signed-off-by: NSzymon Janc <szymon.janc@tieto.com>
Acked-by: NMarcel Holtmann <marcel@holtmann.org>
Signed-off-by: NJohan Hedberg <johan.hedberg@intel.com>