usb: dwc2: gadget: Use true and false for boolean values

Assign true or false to boolean variables instead of an integer value.

This issue was detected with the help of Coccinelle.

Acked-by: John Youn <johnyoun@synopsys.com>
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>