+static const u8 stp_multicast_addr[] = {
+	0x01, 0x80, 0xC2, 0x00, 0x00, 0x00
+};

This probably exists already. Please go looking in include/linux and
include/net.

+static int ksz8863_mdio_read(void *ctx, const void *reg_buf, size_t reg_len,
+			     void *val_buf, size_t val_len)
+{
+	struct ksz_device *dev = (struct ksz_device *)ctx;

ctx is a void *, so you don't need the cast.

+	struct mdio_device *mdev = (struct mdio_device *)dev->priv;

and priv is also probably a void *.

And please fix the reverse christmas tree.

Thanks
	Andrew