Bluetooth: Refactor L2CAP connect rejection to its own function

We'll need to have a separate code path for LE based connection
rejection so it's cleaner to move out the response construction code
into its own function (and later a second one for LE).

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