net: phy: dp83822: Fix null pointer access on DP83825/DP83826 devices

The probe() function is only used for the DP83822 PHY, leaving the
private data pointer uninitialized for the smaller DP83825/26 models.
While all uses of the private data structure are hidden in 82822 specific
callbacks, configuring the interrupt is shared across all models.
This causes a NULL pointer dereference on the smaller PHYs as it accesses
the private data unchecked. Verifying the pointer avoids that.

Fixes: 5dc39fd ("net: phy: DP83822: Add ability to advertise Fiber connection")
Signed-off-by: Andre Kalb <andre.kalb@sma.de>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Link: https://lore.kernel.org/r/Y9FzniUhUtbaGKU7@pc6682
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/phy/dp83822.c

@@ -233,7 +233,8 @@ static int dp83822_config_intr(struct phy_device *phydev)
 				DP83822_ENERGY_DET_INT_EN |
 				DP83822_LINK_QUAL_INT_EN);
 
-		if (!dp83822->fx_enabled)
+		/* Private data pointer is NULL on DP83825/26 */
+		if (!dp83822 || !dp83822->fx_enabled)
 			misr_status |= DP83822_ANEG_COMPLETE_INT_EN |
 				       DP83822_DUP_MODE_CHANGE_INT_EN |
 				       DP83822_SPEED_CHANGED_INT_EN;
@@ -253,7 +254,8 @@ static int dp83822_config_intr(struct phy_device *phydev)
 				DP83822_PAGE_RX_INT_EN |
 				DP83822_EEE_ERROR_CHANGE_INT_EN);
 
-		if (!dp83822->fx_enabled)
+		/* Private data pointer is NULL on DP83825/26 */
+		if (!dp83822 || !dp83822->fx_enabled)
 			misr_status |= DP83822_ANEG_ERR_INT_EN |
 				       DP83822_WOL_PKT_INT_EN;