USB: sierra: fix memory leak in probe error path

commit 084817d7 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>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>