usb: dwc2: fix default value for DMA support

The current defaults for DMA results on a non-DMA platform in a unnecessary
error message:

  Invalid value 0 for param gadget-dma

So fix this by using dma_capable as default value.

Fixes: 9962b62f ("usb: dwc2: Deprecate g-use-dma binding")
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>