USB: serial: fix infinite wait_until_sent timeout

Make sure to handle an infinite timeout (0).

Note that wait_until_sent is currently never called with a 0-timeout
argument due to a bug in tty_wait_until_sent.

Fixes: dcf01050 ("USB: serial: add generic wait_until_sent
implementation")
Cc: stable <stable@vger.kernel.org>	# v3.10
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/serial/generic.c

@@ -258,6 +258,7 @@ void usb_serial_generic_wait_until_sent(struct tty_struct *tty, long timeout)
 	 * character or at least one jiffy.
 	 */
 	period = max_t(unsigned long, (10 * HZ / bps), 1);
+	if (timeout)
 		period = min_t(unsigned long, period, timeout);
 
 	dev_dbg(&port->dev, "%s - timeout = %u ms, period = %u ms\n",
@@ -268,7 +269,7 @@ void usb_serial_generic_wait_until_sent(struct tty_struct *tty, long timeout)
 		schedule_timeout_interruptible(period);
 		if (signal_pending(current))
 			break;
-		if (time_after(jiffies, expire))
+		if (timeout && time_after(jiffies, expire))
 			break;
 	}
 }