sample: usb: Change integration_platforms to platform_allow
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 <cb34e965b6a5a3425eafdbd0feb7e469ae1417ea@nordicsemi.no>

zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,finikorg/zephyr,galak/zephyr,galak/zephyr,finikorg/zephyr,finikorg/zephyr