net: phy: realtek: Use the dummy stubs for MMD register access for rtl8211b

The Ethernet on mpc8315erdb is broken since commit b6b5e8a6
("gianfar: Disable EEE autoneg by default"). The reason is that
even though the rtl8211b doesn't support the MMD extended registers
access, it does return some random values if we trying to access
the MMD register via indirect method. This makes it seem that the
EEE is supported by this phy device. And the subsequent writing to
the MMD registers does cause the phy malfunction. So use the dummy
stubs for the MMD register access to fix this issue.

Fixes: b6b5e8a6

 ("gianfar: Disable EEE autoneg by default")
Signed-off-by: Kevin Hao <haokexin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>