Bluetooth: Update Adv monitor count upon removal

This fixes the count of Adv monitor upon monitor removal.

The following test was performed.
- Start two btmgmt consoles, issue a btmgmt advmon-remove command on one
console and observe a MGMT_EV_ADV_MONITOR_REMOVED event on the other.
Signed-off-by: NMiao-chen Chou <mcchou@chromium.org>
Signed-off-by: NHoward Chung <howardchung@google.com>
Reviewed-by: NAlain Michaud <alainm@chromium.org>
Signed-off-by: NMarcel Holtmann <marcel@holtmann.org>

net/bluetooth/hci_core.c

@@ -3061,6 +3061,7 @@ static int free_adv_monitor(int id, void *ptr, void *data)
 
 	idr_remove(&hdev->adv_monitors_idr, monitor->handle);
 	hci_free_adv_monitor(monitor);
+	hdev->adv_monitors_cnt--;
 
 	return 0;
 }
@@ -3077,6 +3078,7 @@ int hci_remove_adv_monitor(struct hci_dev *hdev, u16 handle)
 
 		idr_remove(&hdev->adv_monitors_idr, monitor->handle);
 		hci_free_adv_monitor(monitor);
+		hdev->adv_monitors_cnt--;
 	} else {
 		/* Remove all monitors if handle is 0. */
 		idr_for_each(&hdev->adv_monitors_idr, &free_adv_monitor, hdev);