usb: gadget: dummy: fix sparse warning

fix the following sparse warning:

drivers/usb/gadget/dummy_hcd.c:1912:12: warning: symbol 'usb3_bos_desc' was not declared. Should it be static?

Acked-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>