kconfig: clean up nested if-conditionals in check_conf()

Unify the outer two if-conditionals into one. This decreases the
indent level by one.

Also, change the if-else blocks:

    if (input_mode == listnewconfig) {
            ...
    } else if (input_mode == helpnewconfig) {
            ...
    } else {
            ...
    }

into the switch statement.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>