arch/powerpc/cpu/mpc8xxx/ddr/ctrl_regs.c: Fix GCC 4.6 build warning

arch/powerpc/cpu/mpc8xxx/ddr/ctrl_regs.c: Fix GCC 4.6 build warning

Fix:

ctrl_regs.c: In function 'set_ddr_sdram_cfg_2':
ctrl_regs.c:641:15: warning: variable 'rcw_en' set but not used [-Wunused-but-set-variable]
ctrl_regs.c: In function 'compute_fsl_memctl_config_regs':
ctrl_regs.c:951:31: warning: array subscript is above array bounds [-Warray-bounds]
ctrl_regs.c:752:34: warning: array subscript is above array bounds [-Warray-bounds]

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>