Refactor iwmbtfw: modularize and de-constify

• Move logic for firmware download into different adapter types to the own functions to allow main() to be simpler
• Use enums/typedefs for exit modes, image types, etc
• Purge most obvious comments: functions are named properly, so most of their invocations self-document the code.

Signed-off-by: Eygene Ryabinkin <rea@FreeBSD.org>
Reviewed by: wulf
MFC after: 1 month

(cherry picked from commit 06969db312022277729dd144e3655a90007306ef)