USB: sierra: fix memory leak in probe error path

commit 084817d79399ab5ccab2f90a148b0369912a8369 upstream.

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

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