net: phy: Have __phy_modify return 0 on success

__phy_modify would return the old value of the register before it was
modified. Thus on success, it does not return 0, but a positive value.
Thus functions using phy_modify, which is a wrapper around
__phy_modify, can start returning > 0 on success, rather than 0. As a
result, breakage has been noticed in various places, where 0 was
assumed.

Code inspection does not find any current location where the return of
the old value is currently used. So have __phy_modify return 0 on
success. When there is a real need for the old value, either a new
accessor can be added, or an additional parameter passed.

Fixes: fea23fb5 ("net: phy: convert read-modify-write to phy_modify()")
Fixes: 2b74e5be ("net: phy: add phy_modify() accessor")
Reported-by: NGeert Uytterhoeven <geert@linux-m68k.org>
Tested-by: NGeert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: NAndrew Lunn <andrew@lunn.ch>
Tested-by: NNiklas Cassel <niklas.cassel@axis.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/phy/phy-core.c

@@ -336,16 +336,15 @@ EXPORT_SYMBOL(phy_write_mmd);
  */
 int __phy_modify(struct phy_device *phydev, u32 regnum, u16 mask, u16 set)
 {
-	int ret, res;
+	int ret;
 
 	ret = __phy_read(phydev, regnum);
-	if (ret >= 0) {
-		res = __phy_write(phydev, regnum, (ret & ~mask) | set);
-		if (res < 0)
-			ret = res;
-	}
+	if (ret < 0)
+		return ret;
 
-	return ret;
+	ret = __phy_write(phydev, regnum, (ret & ~mask) | set);
+
+	return ret < 0 ? ret : 0;
 }
 EXPORT_SYMBOL_GPL(__phy_modify);