USB: adutux: fix up error paths

Make sure to return -ENOMEM on all allocation failures and -EIO on a
string-retrieval error (instead of returning -ENODEV for some such
errors).

Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>