ppc4xx: correct SDRAM controller warning message condition

The message "Truncating memory to %d MiB to fit SDRAM controller limits"
should be displayed only when a user chooses an amount of RAM which
can't be represented by the PPC 4xx SDRAM controller (e.g. 129MB, which
would only be valid if the controller supports a bank size of 1MB).

Signed-off-by: Hollis Blanchard <hollis@penguinppc.org>
