USB: sierra: fix memory leak in probe error path

Move interface data allocation to attach so that it is deallocated on
errors in usb-serial probe.

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