Use QBasicAtomicInt load() and store() instead of implicit casting

Use QBasicAtomicInt load() and store() instead of implicit casting

The implicit casts will be unavailable in the near future.

This is a follow up to commit 7f8472af8c03296be9e9ce50d8e9b3089eedf2f1,
which didn't catch all usages of the implicit cast and assignment
operator.

Change-Id: If05c343e2851b41e4a351c56328c4406c688fa0d
Reviewed-on: http://codereview.qt-project.org/6351
Sanity-Review: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Lars Knoll <lars.knoll@nokia.com>