Bluetooth: L2CAP: Unify code for adding and deleting channels

The code for adding and removing channels is very similar for LE and
BR/EDR so it can be unified and just leave the CID allocation up for the
bearer implementation.

Change-Id: Icf4c66262c4e973039381ebd1861fad4d46e1535
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>