USB: wusb: don't leak urb in certain error cases

Don't leak an urb in wusb_dev_alloc() if the following kmalloc() failed.

Reported-by: Julia Lawall <julia@diku.dk>
Signed-off-by: David Vrabel <david.vrabel@csr.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>