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: 1b09eeb1 ("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>
(cherry picked from commit 0c7ffc977195c1f71c8132eb5616827e589d4a0f)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>