Bluetooth: Clear ack_timer when sending ack

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