usb: dwc2: Disable descriptor dma mode by default

Even though the IP supports Descriptor DMA mode, it does not support SPLIT
transactions in this mode. So the driver, in its currently form, will not
support LS/FS devices when connected to a HS Hub if Descriptor DMA mode is
enabled.

So we should just default to disable descriptor dma mode.

Signed-off-by: Dinh Nguyen <dinguyen@altera.com>
Acked-by: Paul Zimmerman <paulz@synopsys.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>