usb: f_fs: off by one bug in _ffs_func_bind()

commit 0015f915 upstream.

This loop is supposed to set all the .num[] values to -1 but it's off by
one so it skips the first element and sets one element past the end of
the array.

I've cleaned up the loop a little as well.

Fixes: ddf8abd2 ('USB: f_fs: the FunctionFS driver')
Acked-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>
[bwh: Backported to 3.16:
 - Adjust filename, context
 - Add definition of i variable]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

drivers/usb/gadget/f_fs.c

@@ -2355,7 +2355,8 @@ static int _ffs_func_bind(struct usb_configuration *c,
 	const int super = gadget_is_superspeed(func->gadget) &&
 		func->ffs->ss_descs_count;
 
-	int fs_len, hs_len, ret;
+	int fs_len, hs_len, ret, i;
+	struct ffs_ep *eps_ptr;
 
 	/* Make it a single chunk, less management later on */
 	vla_group(d);
@@ -2388,12 +2389,9 @@ static int _ffs_func_bind(struct usb_configuration *c,
 	       ffs->raw_descs_length);
 
 	memset(vla_ptr(vlabuf, d, inums), 0xff, d_inums__sz);
-	for (ret = ffs->eps_count; ret; --ret) {
-		struct ffs_ep *ptr;
-
-		ptr = vla_ptr(vlabuf, d, eps);
-		ptr[ret].num = -1;
-	}
+	eps_ptr = vla_ptr(vlabuf, d, eps);
+	for (i = 0; i < ffs->eps_count; i++)
+		eps_ptr[i].num = -1;
 
 	/* Save pointers
 	 * d_eps == vlabuf, func->eps used to kfree vlabuf later