usb: fix ZLP handling for Variable-length Data Stage

ZLP flag should only be set if less data is sent
than requested by host and the length is a multiple
of wMaxPacketSize. Current implementation does not
check it correctly.

For some platforms like nRF52, this patch will not
be enough to fix the problem. The driver must be informed
about the transfer type before sending the last packet,
without changing the API, it is not possible.

Signed-off-by: Johann Fischer <j.fischer@phytec.de>