an URQ_REQUEST when DMA segments are passed to usbd_start_transfer();
when the request doesn't include the optional data buffer the size of
the transfer (xfer->length) is 0, in which case usbd_transfer() won't
create a DMA map but call usbd_start_transfer() with no DMA segments.
With the previous change this could result in the bus_dmamap_sync()
implementation dereferencing the NULL-pointer passed as the DMA map
argument.
While at it fix what appears to be a typo in usbd_start_transfer();
in order to determine wheter usbd_start_transfer() was called with
DMA segments check whether the number of segments is > 0 rather than
the pointer to them being > 0.
OK'ed by: imp
b92c02c4c7