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/generic.c b/drivers/usb/serial/generic.c
index 583244b..e1d245a 100644
--- a/drivers/usb/serial/generic.c
+++ b/drivers/usb/serial/generic.c
@@ -179,6 +179,7 @@
 	dbg("%s - port %d", __func__, port->number);
 	generic_cleanup(port);
 }
+EXPORT_SYMBOL_GPL(usb_serial_generic_close);
 
 static int usb_serial_multi_urb_write(struct tty_struct *tty,
 	struct usb_serial_port *port, const unsigned char *buf, int count)