UEC FIXED PHY: Determine fixed-phy port using UEC interface name.

Fixed a misunderstanding in the original implementation, 'devnum' that
was used in the cpu/ppc4xx/4xx_enet.c implementation was NOT the
PHY's SMI address, rather it was the number of the MAC interface on
the CPU. The equivalent of this for uec_phy will be the UEC number
stored in mii_info->dev->name. Usage example is updated for uec.

Signed-off-by: Richard Retanubun <RichardRetanubun@RuggedCom.com>
Signed-off-by: Ben Warren <biggerbadderben@gmail.com>