net: usb: switch to dev_get_tstats64 and remove usbnet_get_stats64 alias

Replace usbnet_get_stats64() with new identical core function
dev_get_tstats64() in all users and remove usbnet_get_stats64().

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Acked-by: Bjørn Mork <bjorn@mork.no>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>