usb: dwc3: unmap the proper number of sg entries

num_sgs contains the number of sgs assigned by the gadget.
num_mapped_sgs contains the number of mapped sgs which may differ from
the gadget's values. For dma_unmap_sg() we have to provide the value
which was returned by dma_map_sg().

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>