USB: serial: clean up attribute permissions

Clean up the DEVICE_ATTR usage in the USB serial drivers, making them
more obvious as to the permissions that the sysfs files should be.

Note: ftdi_sio.c still has a DEVICE_ATTR() used, that will have to wait
until after 3.12-rc1 comes out when DEVICE_ATTR_WO() shows up in Linus's
tree.

Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
diff --git a/drivers/usb/serial/bus.c b/drivers/usb/serial/bus.c
index f053b30..0ce51f2 100644
--- a/drivers/usb/serial/bus.c
+++ b/drivers/usb/serial/bus.c
@@ -38,15 +38,14 @@
 	return 0;
 }
 
-static ssize_t show_port_number(struct device *dev,
+static ssize_t port_number_show(struct device *dev,
 				struct device_attribute *attr, char *buf)
 {
 	struct usb_serial_port *port = to_usb_serial_port(dev);
 
 	return sprintf(buf, "%d\n", port->port_number);
 }
-
-static DEVICE_ATTR(port_number, S_IRUGO, show_port_number, NULL);
+static DEVICE_ATTR_RO(port_number);
 
 static int usb_serial_device_probe(struct device *dev)
 {