[WATCHDOG] pcwd_usb.c: fix a NULL pointer dereference

The Coverity checker noted that this resulted in a NULL pointer
reference if we were coming from

        if (usb_pcwd == NULL) {
                printk(KERN_ERR PFX "Out of memory\n");
                    goto error;
        }

Signed-off-by: Adrian Bunk <bunk@stusta.de>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>
Signed-off-by: Andrew Morton <akpm@osdl.org>