Bluetooth: L2CAP: Make l2cap_chan_add initialize the destroy callback

This is cleaner and safer as destroy is garanteed to be initialized once
it is added to the list of channels.

Change-Id: Iae674f6e25527d71c03c5903337b0bd18c3dd0be
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>