Bluetooth: Simplify the ERTM ack timeout

Since l2cap_send_ack can trigger extra actions like sending iframes,
don't call it.  Just send an RR or RNR frame if an ack needs sending.

Signed-off-by: Mat Martineau <mathewm@codeaurora.org>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>