usb: dwc2: gadget: fix default value for gadget-dma-desc

The current default for gadget DMA descriptor results on bcm2835 in a
unnecessary error message:

  Invalid value 1 for param gadget-dma-desc

So fix this by using hw->dma_desc_enable as default value.

Fixes: dec4b556 ("usb: dwc2: gadget: Add descriptor DMA parameter")
Acked-by: John Youn <johnyoun@synopsys.com>
Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>