QNdefNfcSmartPosterRecord: fix operator=()

The assignemnt operator only assigned the class' own d-pointer, but
totally ignored the fact that the parent class QNdefRecord has its
own d-pointer.
As a result, all methods that were using the base class' d-ptr returned
outdated results.

For example, the tests in this patch failed because they kept returning
the old payload.

Fix by explicitly calling QNdefRecord::operator=().

Pick-to: 6.12 6.8
Change-Id: I72f292ef69e5ff56ac80ce143a55e98140f6084b
Reviewed-by: Mate Barany <mate.barany@qt.io>