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

In the Linux kernel, the following vulnerability has been resolved:

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.