[2.1-beta-2] Fix checkbox width in product data meta box

While bcee9b4a05604cad2619dacecd2287eb64eac737 fixes the width of checkboxes for narrow screens, the checkboxes seem to inherit a `width: 95%` property from core WP styles for wider width (see screenshot below). This PR hopefully fixes this issue for good across all screen widths - fingers crossed :)

![checkbox](https://f.cloud.github.com/assets/148775/1737081/a792de1e-636c-11e3-9940-c2782a75dbf8.png)