ppc4xx: Fix problem with DIMMs with 8 banks in 44x_spd_ddr2.c

This patch fixes a problem with DIMMs that have 8 banks. Now the
MCIF0_MBxCF register will be setup correctly for this setup too.

This was noticed with the 512MB DIMM on Canyonlands/Glacier.

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