ksz9477: MMD indirect access check fix

Instead of verifying MMD indirect access by checking
EEE ADVERTISEMENT register has value 0x6 (which may be changed),
the verification is done by write and check LED MODE register.
First write value Single-LED mode (0x10), then read the value back
to verify value matches with the written one and then finally write
back the original default value (0x00).

drivers/net/ksz9477.c

@@ -638,15 +638,46 @@ int ksz9477_init(ksz9477_port_t master_port)
     }
 
   /* Check that indirect access to PHY MMD works.
-   * Default value of MMD EEE ADVERTISEMENT REGISTER is 0x6
+   * Write LED mode to single-LED mode and verify access by
+   * reading back the value.
    */
 
+  /* Set LED mode to sigle-LED mode */
+
+  regval16 = 0x10;
+  ret = ksz9477_mmd_write_indirect(KSZ9477_PORT_PHY1,
+                                   KSZ9477_MMD_DEV_LED_MODE,
+                                   0x00, &regval16, 1);
+  if (ret != OK)
+    {
+      nerr("MMD access failure. Failed write 0x%04x to "
+           "LED_MODE register: ret %d\n", regval16, ret);
+      return ret ? ret : -EINVAL;
+    }
+
+  /* Verify read returns Single-LED mode (0x10) */
+
   ret = ksz9477_mmd_read_indirect(KSZ9477_PORT_PHY1,
-                                  KSZ9477_MMD_DEV_EEE_ADVERTISEMENT,
-                                  0x3c, &regval16, 1);
-  if (ret != OK || regval16 != 0x6)
+                                  KSZ9477_MMD_DEV_LED_MODE,
+                                  0x00, &regval16, 1);
+
+  if (ret != OK || regval16 != 0x10)
+    {
+      nerr("MMD access failure. Failed to verify LED_MODE "
+           "register value 0x10: ret %d, regval %04x\n", ret, regval16);
+      return ret ? ret : -EINVAL;
+    }
+
+  /* Set back to default value (0x00) */
+
+  regval16 = 0x00;
+  ret = ksz9477_mmd_write_indirect(KSZ9477_PORT_PHY1,
+                                   KSZ9477_MMD_DEV_LED_MODE,
+                                   0x00, &regval16, 1);
+  if (ret != OK)
     {
-      nerr("MMD access failure, ret %d\n", ret);
+      nerr("MMD access failure. Failed to write 0x%04x to "
+           "LED_MODE register: ret %d\n", regval16, ret);
       return ret ? ret : -EINVAL;
     }