USB: omninet: fix write_room

Fix regression introduced by commit 507ca9b ([PATCH] USB: add
ability for usb-serial drivers to determine if their write urb is
currently being used.) which inverted the logic in write_room so that it
returns zero when the write urb is actually free.

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

drivers/usb/serial/omninet.c

@@ -290,7 +290,7 @@ static int omninet_write_room(struct tty_struct *tty)
 	int room = 0; /* Default: no room */
 
 	/* FIXME: no consistent locking for write_urb_busy */
-	if (wport->write_urb_busy)
+	if (!wport->write_urb_busy)
 		room = wport->bulk_out_size - OMNINET_HEADERLEN;
 
 	dbg("%s - returns %d", __func__, room);