net: phy: qcom: at803x: fix kernel panic with at8031_probe

On reworking and splitting the at803x driver, in splitting function of
at803x PHYs it was added a NULL dereference bug where priv is referenced
before it's actually allocated and then is tried to write to for the
is_1000basex and is_fiber variables in the case of at8031, writing on
the wrong address.

Fix this by correctly setting priv local variable only after
at803x_probe is called and actually allocates priv in the phydev struct.
Reported-by: William Wortel <wwortel@dorpstraat.com>
Cc: <stable@vger.kernel.org>
Fixes: 25d2ba94 ("net: phy: at803x: move specific at8031 probe mode check to dedicated probe")
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://lore.kernel.org/r/20240325190621.2665-1-ansuelsmth@gmail.comSigned-off-by: Paolo Abeni <pabeni@redhat.com>

drivers/net/phy/qcom/at803x.c

@@ -797,7 +797,7 @@ static int at8031_parse_dt(struct phy_device *phydev)
 
 static int at8031_probe(struct phy_device *phydev)
 {
-	struct at803x_priv *priv = phydev->priv;
+	struct at803x_priv *priv;
 	int mode_cfg;
 	int ccr;
 	int ret;
@@ -806,6 +806,8 @@ static int at8031_probe(struct phy_device *phydev)
 	if (ret)
 		return ret;
 
+	priv = phydev->priv;
+
 	/* Only supported on AR8031/AR8033, the AR8030/AR8035 use strapping
 	 * options.
 	 */