lib: sbi: Simplify prints

When doing width = width - strlen(string) in prints there is no need
to consider the case that witdh may be less than 0. This is because
the code to do filling needs to be executed under the condition that
width > 0.

Signed-off-by: Xiang W <wxjstz@126.com>
Reviewed-by: Anup Patel <anup@brainfault.org>