usb: dwc3: gadget: fix DMA offset calculation

Fix offset calculation in dwc3_trb_dma_offset()

Signed-off-by: Paul Zimmerman <paulz@synopsys.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>