usbnet: fix the indentation of one code snippet

Every line of code should start with tab (8 characters)
Signed-off-by: Dongliang Mu <mudongliangabcd@gmail.com>
Link: https://lore.kernel.org/r/20210123051102.1091541-1-mudongliangabcd@gmail.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>