powerpc/freescale: Change the return value of mac_read_from_eeprom()

powerpc/freescale: Change the return value of mac_read_from_eeprom()

The return value has not been checked by its caller, until recent change
of using generic board architecture. The error of this function is not
critical enough to hang the system. Printing the warning message is enough
to catch user's attention. U-boot should continue to boot to give user
a chance to fix the EEPROM. Chaning the return value to 0 to avoid hanging
in the board_init_r().

Signed-off-by: York Sun <yorksun@freescale.com>