Set the length of isochronous transfers as the sum of the frames lengths.

Set the length of isochronous transfers as the sum of the frames lengths.

This reduces differences between non-isoch and isoch transfers submissions,
makes the generic DMA buffer overrun check work with isoch transfers and
will allow some code simplifications in HC drivers.

Since short-transfers were never checked for isoch transfers, we now need to
pass the USBD_SHORT_XFER_OK flag to not change this behavior.  This might be
revisited later.

ok ratchov@