Bluetooth: L2CAP: Refactor l2cap core types

Refactors L2CAP core data structures and modifies functions that uses them.
Now we can have separate L2CAP feature channel objects for LE and BR/EDR
transport. This's also a ground for make L2CAP CoC API to be common
for both transports.

Change-Id: Ic1197b0e3c4f505764b23fb9c7eb27123a44a675
Signed-off-by: Arkadiusz Lichwa <arkadiusz.lichwa@tieto.com>