usb: gadget: f_fs: Only return delayed status when len is 0

Hi Felipe,

I noticed this wasn't queued up for 4.18. Do you think there is anything I need to do to get this patch set into 4.19? Also, can we at least add just this patch ('usb: gadget: f_fs: Only return delayed status when len is 0') to 4.18 as functionfs control requests won't work without it?