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>