thunderbolt: Expose usb4_port_index() to other modules

Make usb4_port_index() available to other files in the driver, rename
and add function documentation.

Signed-off-by: Alan Borzeszkowski <[e-mail]>
Reviewed-by: Heikki Krogerus <[e-mail]>
Signed-off-by: Mika Westerberg <[e-mail]>

drivers/thunderbolt/tb.h

···
 struct usb4_port *usb4_port_device_add(struct tb_port *port);
 void usb4_port_device_remove(struct usb4_port *usb4);
 int usb4_port_device_resume(struct usb4_port *usb4);
+int usb4_port_index(const struct tb_switch *sw, const struct tb_port *port);
 
 static inline bool usb4_port_device_is_offline(const struct usb4_port *usb4)
 {

drivers/thunderbolt/usb4.c

···
 	return status ? -EIO : 0;
 }
 
-static int usb4_port_idx(const struct tb_switch *sw, const struct tb_port *port)
+/**
+ * usb4_port_index() - Finds matching USB4 port index
+ * @sw: USB4 router
+ * @port: USB4 protocol or lane adapter
+ *
+ * Finds matching USB4 port index (starting from %0) that given @port goes
+ * through.
+ */
+int usb4_port_index(const struct tb_switch *sw, const struct tb_port *port)
 {
 	struct tb_port *p;
 	int usb4_idx = 0;
···
 struct tb_port *usb4_switch_map_pcie_down(struct tb_switch *sw,
 					  const struct tb_port *port)
 {
-	int usb4_idx = usb4_port_idx(sw, port);
+	int usb4_idx = usb4_port_index(sw, port);
 	struct tb_port *p;
 	int pcie_idx = 0;
 
···
 struct tb_port *usb4_switch_map_usb3_down(struct tb_switch *sw,
 					  const struct tb_port *port)
 {
-	int usb4_idx = usb4_port_idx(sw, port);
+	int usb4_idx = usb4_port_index(sw, port);
 	struct tb_port *p;
 	int usb_idx = 0;