hdlx_x25: Fix backwards compat test.

drivers/net/wan/hdlc_x25.c: In function ‘x25_ioctl’:
drivers/net/wan/hdlc_x25.c:256:7: warning: suggest parentheses around assignment used as truth value [-Wparentheses]
  256 |   if (ifr->ifr_settings.size = 0) {
      |       ^~~
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/wan/hdlc_x25.c

@@ -253,7 +253,7 @@ static int x25_ioctl(struct net_device *dev, struct ifreq *ifr)
 			return -EBUSY;
 
 		/* backward compatibility */
-		if (ifr->ifr_settings.size = 0) {
+		if (ifr->ifr_settings.size == 0) {
 			new_settings.dce = 0;
 			new_settings.modulo = 8;
 			new_settings.window = 7;