usb: gadget: f_fs: fix sparse warning

use NULL when returning NULL pointers, not 0.

Signed-off-by: Felipe Balbi <balbi@ti.com>
diff --git a/drivers/usb/gadget/f_fs.c b/drivers/usb/gadget/f_fs.c
index 12a64e1..306a2b5 100644
--- a/drivers/usb/gadget/f_fs.c
+++ b/drivers/usb/gadget/f_fs.c
@@ -1137,7 +1137,7 @@
 {
 	struct ffs_data *ffs = kzalloc(sizeof *ffs, GFP_KERNEL);
 	if (unlikely(!ffs))
-		return 0;
+		return NULL;
 
 	ENTER();