Bluetooth: L2CAP: Refactor channel context by ident

Moves 'ident' member so far included in bt_l2cap_le_chan context only
to common bt_l2cap_chan context. The change is driven by sharing ability
to store/restore 'ident' on both transports and makes use of it only if
CoC is configured. For default configuration there's channel addition
helper to be used internally by dedicated macros managing channel
objects enlisted in connection tracker list.
The common member is used in matching outstanding connection responses
on LE and to prepare a ground for BR/EDR transport context when
there'll be a need to match response to original incoming connection
request after in-between security changes that can happen.

Change-Id: I1a4cad7cd53c74774604a0e9794607081a8b6e80
Signed-off-by: Arkadiusz Lichwa <arkadiusz.lichwa@tieto.com>