init/main.c: Minor cleanup for the setup_command_line() function

This is just a minor cleanup to make the code look a bit cleaner.

Link: https://lore.kernel.org/all/20240412081733.35925-3-ytcoode@gmail.com/

Signed-off-by: Yuntao Wang <ytcoode@gmail.com>
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>