sample: usb: Change integration_platforms to platform_allow

The sample proviades required overlays only for 3 platforms.
Therefore, flash scenarios can only work on those 3 platforms.
To reflect this, platform_allow has to be used instead of
integration_platforms, so boards without overlays are not picked up
into a scope.

Signed-off-by: Maciej Perkowski <Maciej.Perkowski@nordicsemi.no>