usb: device: move the content of usb_msc.h to implementation file

The usb_msc.h header does not provide any API. The content is only
used by the MSC implementation for the current USB device stack and
is not required for any use of MSC by the application.
The content has no proper namespace and must not be reused for
anything else in (new) USB support.

Signed-off-by: Johann Fischer <johann.fischer@nordicsemi.no>