Bluetooth: Don't send RNR immediately when entering local busy

There is no need to send RNR immediately when entring local busy.
Also upper layer might clear local busy condition before ack timer
expires saving few cycles for sending RNR.

This also prevents sending two RNRs in some cases where sending one
would be enough i.e received N I-frame can trigger local busy
(sending RNR acking up to N-1 I-frame) and later sending ack (RNR
acking up to N I-frame).

This was affecting TC_ERM_BV_07_C and TC_ERM_BV_22_C with some non
default channel parameters (tx window and receiving buffer sizes).

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>