device/dram/ddr3.c: Fix sizeof on array func param overflow

The sizeof on array function parameter will return size of 'u8 *'
instead of 'spd_raw_data' (aka 'u8 [256]' leading to an overflow.

Found-by: Clang
Change-Id: I78e113a640b2953c853eb43bd6874e4694260b1f
Signed-off-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Reviewed-on: http://review.coreboot.org/7353
Tested-by: build bot (Jenkins)
Reviewed-by: Vladimir Serbinenko <phcoder@gmail.com>