include: sbi: No need to pack struct sbi_scratch

All members in struct sbi_scrach are of machine word size so no need
to pack this structure. This also allows GCC to generate more optimized
instruction sequence when accessing members of struct sbi_scratch.

Reported-by: Paul Campbell <taniwha@gmail.com>
Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Atish Patra <atish.patra@wdc.com>