usb: gadget: f_dfu: write req->actual bytes

If last packet is short, we shouldn't write req->length bytes to
non-volatile media, we should write only what's available to us, which
is held in req->actual.

Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>