Bluetooth: Handle LE L2CAP signalling in its own function

The LE L2CAP signalling channel follows its own rules and will continue
to evolve independently from the BR/EDR signalling channel. Therefore,
it makes sense to have a clear split from BR/EDR by having a dedicated
function for handling LE signalling commands.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>