USB: serial: refactor generic close

Export usb_serial_generic_close so that drivers can easily kill the read
and write urb and make sure that the write fifo is reset.

Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

diff --git a/drivers/usb/serial/belkin_sa.c b/drivers/usb/serial/belkin_sa.c
index 1295e44..bdab31a 100644
--- a/drivers/usb/serial/belkin_sa.c
+++ b/drivers/usb/serial/belkin_sa.c
@@ -246,9 +246,7 @@
 {
 	dbg("%s port %d", __func__, port->number);
 
-	/* shutdown our bulk reads and writes */
-	usb_kill_urb(port->write_urb);
-	usb_kill_urb(port->read_urb);
+	usb_serial_generic_close(port);
 	usb_kill_urb(port->interrupt_in_urb);
 } /* belkin_sa_close */