usb: gadget: dummy_hcd: replace timeval with timespec64

The millisecond of the last second will be normal if tv_sec is
overflowed. But for y2038 consistency and demonstration purpose,
and avoiding further risks, we need to remove 'timeval' in this
driver, to avoid similair problems.

Signed-off-by: Pingbo Wen <pingbo.wen@linaro.org>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>