Bluetooth: L2CAP: Move functions in preparation for queuing

This moves necessary functions that will be needed for queuing packets
while waiting for more credits.

Jira: ZEP-1776

Change-Id: I030c696d432ec5be1b8e6b649e953da145929777
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>