drivers: usb: Mark function as static in metro-usb.c

Mark function metrousb_is_unidirectional_mode() in serial/metro-usb.c
because it is not used outside this file.

This eliminates the following warning in serial/metro-usb.c:
drivers/usb/serial/metro-usb.c:57:12: warning: no previous prototype for ‘metrousb_is_unidirectional_mode’ [-Wmissing-prototypes]
Signed-off-by: Rashika Kheria <rashika.kheria@gmail.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/serial/metro-usb.c

@@ -54,7 +54,7 @@ MODULE_DEVICE_TABLE(usb, id_table);
 #define UNI_CMD_OPEN	0x80
 #define UNI_CMD_CLOSE	0xFF
 
-inline int metrousb_is_unidirectional_mode(struct usb_serial_port *port)
+static inline int metrousb_is_unidirectional_mode(struct usb_serial_port *port)
 {
 	__u16 product_id = le16_to_cpu(
 		port->serial->dev->descriptor.idProduct);