Bluetooth: Handle all cases of receipt of RNR-frames into L2CAP

We weren't handling the receipt under SREJ_SENT state table.
It also introduce l2cap_send_srejtail(). It will be used in the nexts
commits too.

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>