iwlwifi: reorganize firmware API

Apart from DVM, all firmware uses the same base API, and there's
code outside iwlmvm that needs to interact with it. Reflect this
in the source better and reorganize the firmware API to a new
fw/api/ directory.

While at it, split the already pretty large fw-api.h file into a
number of smaller files, going from almost 3k lines in there to
a maximum number of lines less than 1k.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>