net: dsa: realtek: fix out-of-bounds access

In the Linux kernel, the following vulnerability has been resolved: net: dsa: realtek: fix out-of-bounds access The probe function sets priv->chip_data to (void *)priv + sizeof(*priv) with the expectation that priv has enough trailing space. However, only realtek-smi actually allocated this chip_data space. Do likewise in realtek-mdio to fix out-of-bounds accesses. These accesses likely went unnoticed so far, because of an (unused) buf[4096] member in struct realtek_priv, which caused kmalloc to round up the allocated buffer to a big enough size, so nothing of value was overwritten. With a different allocator (like in the barebox bootloader port of the driver) or with KASAN, the memory corruption becomes quickly apparent.