x86/microcode/intel: Rework intel_find_matching_signature()

Take a cpu_signature argument and work from there. Move the match()
helper next to the callsite as there is no point for having it in
a header.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20231002115902.797820205@linutronix.de