usb: gadget.h: be consistent at kernel doc macros

There's one value that use spaces instead of tabs to ident.
That causes the following warning:

./include/linux/usb/gadget.h:193: ERROR: Unexpected indentation.
Signed-off-by: NMauro Carvalho Chehab <mchehab@s-opensource.com>
Acked-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: NJonathan Corbet <corbet@lwn.net>