lib: sbi: Fix bug in strncmp function when count is 0

No need to compare characters when the count turns to 0.
Fix the issue in sbi_strncmp.

Signed-off-by: Dong Du <Dd_nirvana@sjtu.edu.cn>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Anup Patel <anup.patel@wdc.com>