Add double buffering routines

Signed-off-by: Manuel Odendahl <wesen@ruinwesen.com>
Rename USB_EP_EP_KIND to USB_EP_EP_KIND_DBL_BUF

Signed-off-by: Manuel Odendahl <wesen@ruinwesen.com>
Refactor setting a USB EP rx count

Signed-off-by: Manuel Odendahl <wesen@ruinwesen.com>