Bluetooth: Fix notifying discovery state upon reset

When HCI_Reset is issued the discovery state is assumed to be stopped.
The hci_cc_reset() handler was trying to set the state but it was doing
it without using the hci_discovery_set_state() function. Because of this
e.g. the mgmt Discovering event could go without being sent. This patch
fixes the code to use the hci_discovery_set_state() function instead of
just blindly setting the state value.

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