hw/net/xilinx_axienet: Prevent writes to PHY Identification registers

There are other registers in the PHY model which should be partially or
entirely read-only, but this solves the immediate issue.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/3297

Signed-off-by: Jim MacArthur <jim.macarthur@linaro.org>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@amd.com>
Message-ID: <20260305-xilinx-phy-protection-v1-1-ffc9edd84e58@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>

hw/net/xilinx_axienet.c

@@ -141,6 +141,10 @@ tdk_write(struct PHY *phy, unsigned int req, unsigned int data)
     regnum = req & 0x1f;
     DPHY(qemu_log("%s reg[%d] = %x\n", __func__, regnum, data));
     switch (regnum) {
+        case 2:
+        case 3:
+            /* Writes to PHY Identification registers are disallowed */
+            break;
         default:
             phy->regs[regnum] = data;
             break;