USB: serial: keyspan_pda: fix stalled writes

Make sure to clear the write-busy flag also in case no new data was
submitted due to lack of device buffer space so that writing is
resumed once space again becomes available.

Fixes: 507ca9bc ("[PATCH] USB: add ability for usb-serial drivers to determine if their write urb is currently being used.")
Cc: stable <stable@vger.kernel.org>     # 2.6.13
Acked-by: NSebastian Andrzej Siewior <bigeasy@linutronix.de>
Reviewed-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: NJohan Hovold <johan@kernel.org>

drivers/usb/serial/keyspan_pda.c

@@ -548,7 +548,7 @@ static int keyspan_pda_write(struct tty_struct *tty,
 
 	rc = count;
 exit:
-	if (rc < 0)
+	if (rc <= 0)
 		set_bit(0, &port->write_urbs_free);
 	return rc;
 }