It seems that there is problem with this application. You could use "mdio write" command in u-boot, please refer to the following example.
=> mdio write FM1@DTSEC3 0xd 0x7
=> mdio write FM1@DTSEC3 0xe 0x3c
=> mdio write FM1@DTSEC3 0xd 0x4007
=> mdio read FM1@DTSEC3 0xe
Reading from bus FSL_MDIO0
PHY at address 1:
14 - 0x6
=> mdio write FM1@DTSEC3 0xe 0x0
=> mdio read FM1@DTSEC3 0xe
Reading from bus FSL_MDIO0
PHY at address 1:
14 - 0x0
=>