usb: cdns3: gadget: using correct sg operations

It needs to use request->num_mapped_sgs to indicate mapped sg number,
the request->num_sgs is the sg number before the mapping. These two
entries have different values for the platforms which iommu or
swiotlb is used. Besides, it needs to use correct sg APIs for
mapped sg list for TRB assignment.

Signed-off-by: Peter Chen <peter.chen@nxp.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>