Bluetooth: L2CAP: Log state transitions

This adds a common function to log state transitions including the caller
function and line in case something goes wrong:

[bt] [DBG] bt_l2cap_chan_disconnect: (0x0011b14c) chan 0x001199c0 scid 0x0040 dcid 0x0040
[bt] [DBG] bt_l2cap_chan_set_state_debug: (0x0011b14c) chan 0x001199c0 psm 0x0080 connect -> disconnect
[bt] [WRN] bt_l2cap_chan_set_state_debug: bt_l2cap_chan_disconnect()1562: invalid transition

Change-Id: I246a9004a04d93a38b8c7f7633705f6c191698b2
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>