iwlwifi: create new subdirectory for FW interaction

There's a lot of mvm code that really should be more generic
and part of the iwlwifi module. Start by making a place to
keep such code - in the new "fw" subdirectory - and already
move the firmware related header files there.

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