ppc4xx: EMAC: Fix 405EZ fifo size setup in EMAC_MR1

The 405EZ only supports 512 bytes of rx-/tx-fifo EMAC sizes. But
currently 4k/2k is configured. This patch fixes this issue.

Thanks to Thomas Kindler <tkindler@lenord.de> for pointing this out.

Signed-off-by: Stefan Roese <sr@denx.de>