usb: dwc2: gadget: use | instead of + for bitmasks

It's just a lot clearer to use | operator instead of
+ operator.

Caught by coccicheck:

	"
	drivers/usb/dwc2/gadget.c:2883:26-27: WARNING: sum of probable
	bitmasks, consider |
	"

Cc: John Youn <John.Youn@synopsys.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>