macb: Fix mii_phy_read and mii_phy_write functions

Enabling CONFIG_CMD_MII in AVR32 boards was not possible due to
compile errors.

This patch fixes miiphy_read and miiphy_write functions and
registers them properly.

Signed-off-by: Semih Hazar <semih.hazar@indefia.com>
Signed-off-by: Ben Warren <biggerbadderben@gmail.com>