wifi: wfx: prevent underflow in wfx_send_pds()

This does a "chunk_len - 4" subtraction later when it calls:

	ret = wfx_hif_configuration(wdev, buf + 4, chunk_len - 4);

so check for "chunk_len" is less than 4.

Fixes: dcbecb49 ("staging: wfx: allow new PDS format")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/Yv8eX7Xv2ubUOvW7@kili

drivers/net/wireless/silabs/wfx/main.c

@@ -181,7 +181,7 @@ int wfx_send_pds(struct wfx_dev *wdev, u8 *buf, size_t len)
 	while (len > 0) {
 		chunk_type = get_unaligned_le16(buf + 0);
 		chunk_len = get_unaligned_le16(buf + 2);
-		if (chunk_len > len) {
+		if (chunk_len < 4 || chunk_len > len) {
 			dev_err(wdev->dev, "PDS:%d: corrupted file\n", chunk_num);
 			return -EINVAL;
 		}