korad-kaxxxxp: eliminate numeric model ID (table index) in scan

korad-kaxxxxp: eliminate numeric model ID (table index) in scan

Rephrase the successful lookup of a supported model by means of a
pointer to a models[] entry. Which eliminates the numeric index,
including the "sentinel" value which points outside of the table.

Use a signed integer for iteration over table items. Don't break
text lines between vendor and model fields, these fields form a
logical group from the user's perspective.