usb: gadget: f_fs: fix sparse warning

use NULL when returning NULL pointers, not 0.

Signed-off-by: Felipe Balbi <[e-mail]>

drivers/usb/gadget/f_fs.c

···
 {
 	struct ffs_data *ffs = kzalloc(sizeof *ffs, GFP_KERNEL);
 	if (unlikely(!ffs))
-		return 0;
+		return NULL;
 
 	ENTER();