ibm_newemac: Add support for 460EX/GT-type MAL rx-channel handling

On some 4xx PPC's (e.g. 460EX/GT), the rx channel number is a multiple
of 8 (e.g. 8 for EMAC1, 16 for EMAC2), but enabling in MAL_RXCASR needs
the divided by 8 value for the bitmask.

Signed-off-by: Stefan Roese <sr@denx.de>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>