net: dsa: b53: clean up if() condition to be more readable

I've stared at this if() statement for a while trying to work out if
it really does correspond with the comment above, and it does seem to.
However, let's make it more readable and phrase it in the same way as
the comment.

Also add a FIXME into the comment - we appear to deny Gigabit modes for
802.3z interface modes, but 802.3z interface modes only operate at
gigabit and above.

Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>                            Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>

Author: Russell King (Oracle)

drivers/net/dsa/b53/b53_common.c

@@ -1327,11 +1327,14 @@ void b53_phylink_validate(struct dsa_switch *ds, int port,
 
 	/* With the exclusion of 5325/5365, MII, Reverse MII and 802.3z, we
 	 * support Gigabit, including Half duplex.
+	 *
+	 * FIXME: this is weird - 802.3z is always Gigabit, but we exclude
+	 * it here. Why? This makes no sense.
 	 */
-	if (state->interface != PHY_INTERFACE_MODE_MII &&
-	    state->interface != PHY_INTERFACE_MODE_REVMII &&
-	    !phy_interface_mode_is_8023z(state->interface) &&
-	    !(is5325(dev) || is5365(dev))) {
+	if (!(state->interface == PHY_INTERFACE_MODE_MII ||
+	      state->interface == PHY_INTERFACE_MODE_REVMII ||
+	      phy_interface_mode_is_8023z(state->interface) ||
+	      is5325(dev) || is5365(dev))) {
 		phylink_set(mask, 1000baseT_Full);
 		phylink_set(mask, 1000baseT_Half);
 	}