Bluetooth: L2CAP: Implement bt_l2cap_br_connected() notifier

Adds implementation of the BR/EDR connection L2CAP notifier. It makes,
for each registered BR/EDR fixed channel, basic validation and channel
initialization using two local helper functions.

Change-Id: I6393a43b25ff0d9f23deaf93562add08861ea1d2
Signed-off-by: Arkadiusz Lichwa <arkadiusz.lichwa@tieto.com>