usb: dwc3: gadget: increment request->actual once

When using SG lists, we would end up setting
request->actual to:

	num_mapped_sgs * (request->length - count)

Let's fix that up by incrementing request->actual
only once.

Cc: <stable@vger.kernel.org>
Reported-by: Brian E Rogers <brian.e.rogers@intel.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>