Bluetooth: Clean up l2cap_chan_add

Change elseif to switch. This make sense even more with following
patches which otherwise have to add more elseifs statements.

Signed-off-by: Andrei Emeltchenko <andrei.emeltchenko@intel.com>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>