40x_spd_sdram.c: Fix GCC 4.6 build warnings

Fix:
40x_spd_sdram.c: In function 'spd_sdram':
40x_spd_sdram.c:137:6: warning: variable 'sdram0_b3cr' set but not used [-Wunused-but-set-variable]
40x_spd_sdram.c:136:6: warning: variable 'sdram0_b2cr' set but not used [-Wunused-but-set-variable]
40x_spd_sdram.c:129:6: warning: variable 'sdram0_ecccfg' set but not used [-Wunused-but-set-variable]

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