net: phy: test the right variable in phy_write_mmd()

This is a copy and paste buglet.  We meant to test for ->write_mmd but
we test for ->read_mmd.

Fixes: 1ee6b9bc6206 ("net: phy: make phy_(read|write)_mmd() generic MMD accessors")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/phy/phy-core.c

@@ -76,7 +76,7 @@ int phy_write_mmd(struct phy_device *phydev, int devad, u32 regnum, u16 val)
 	if (regnum > (u16)~0 || devad > 32)
 		return -EINVAL;
 
-	if (phydev->drv->read_mmd) {
+	if (phydev->drv->write_mmd) {
 		ret = phydev->drv->write_mmd(phydev, devad, regnum, val);
 	} else if (phydev->is_c45) {
 		u32 addr = MII_ADDR_C45 | (devad << 16) | (regnum & 0xffff);