board/sacsng/sacsng.c: Fix GCC 4.6 build warning

Fix:
sacsng.c: In function 'initdram':
sacsng.c:180:9: warning: variable 'spd_size' set but not used
[-Wunused-but-set-variable]

Signed-off-by: Wolfgang Denk <wd@denx.de>
Cc: Jerry Van Baren <gerald.vanbaren@smiths-aerospace.com>