lib: Remove fp_dev->priv

Not precise enough, and it was only used in upekts to get/set the
instance private struct and keep it around, so add a new, more precisely
defined field to fp_dev for that.

libfprint/fp_internal.h

@@ -68,15 +68,14 @@ struct fp_dev {
 
 	/* only valid if drv->type == DRIVER_IMAGING */
 	struct fp_img_dev *img_dev;
+	/* Link to the instance specific struct */
+	void *instance_data;
 
 	int nr_enroll_stages;
 
 	/* FIXME: This will eventually have a bus type */
 	libusb_device_handle *udev;
 
-	/* FIXME: Remove, should be the driver private data */
-	void *priv;
-
 	/* read-only to drivers */
 	struct fp_print_data *verify_data;