USB: serial: export usb_serial_generic_chars_in_buffer

Export generic chars_in_buffer implementation so it can be used in
subdrivers in combination with checks of any hardware buffers.
Signed-off-by: NJohan Hovold <jhovold@gmail.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/serial/generic.c

@@ -262,6 +262,7 @@ int usb_serial_generic_chars_in_buffer(struct tty_struct *tty)
 	dev_dbg(&port->dev, "%s - returns %d\n", __func__, chars);
 	return chars;
 }
+EXPORT_SYMBOL_GPL(usb_serial_generic_chars_in_buffer);
 
 static int usb_serial_generic_submit_read_urb(struct usb_serial_port *port,
 						int index, gfp_t mem_flags)