hw/net: cadence_gem: Fix MDIO_OP_xxx values

Testing upstream U-Boot with 'sifive_u' machine we see:

  => dhcp
  ethernet@10090000: PHY present at 0
  Could not get PHY for ethernet@10090000: addr 0
  phy_connect failed

This has been working till QEMU 8.1 but broken since QEMU 8.2.

Fixes: 1b09eeb ("hw/net/cadence_gem: use FIELD to describe PHYMNTNC register fields")
Reported-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Bin Meng <bmeng@tinylab.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Tested-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

hw/net/cadence_gem.c

@@ -199,8 +199,8 @@ REG32(PHYMNTNC, 0x34) /* Phy Maintenance reg */
     FIELD(PHYMNTNC, PHY_ADDR, 23, 5)
     FIELD(PHYMNTNC, OP, 28, 2)
     FIELD(PHYMNTNC, ST, 30, 2)
-#define MDIO_OP_READ    0x3
-#define MDIO_OP_WRITE   0x2
+#define MDIO_OP_READ    0x2
+#define MDIO_OP_WRITE   0x1
 
 REG32(RXPAUSE, 0x38) /* RX Pause Time reg */
 REG32(TXPAUSE, 0x3c) /* TX Pause Time reg */