Bluetooth: Add discovery state tracking

This patch adds proper state tracking to the device discovery process.
This makes it possible to return appropriate errors when trying to stop
a non-active discovery or start discovery when it is already ongoing.
Once name resolving is implemented this also makes it possible to know
what the right action to do is when a remote name lookup is cancelled.

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