Bluetooth: Refactor stop_discovery

This patch does a trivial code refactoring in stop_discovery
function by using a switch statement instead of an if-return-else
approach.

Signed-off-by: Andre Guedes <andre.guedes@openbossa.org>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Gustavo Padovan <gustavo@padovan.org>