iwlwifi: move firmware file format definitions to correct header

The firmware file-format is described in iwl-fw-file.h, while the
internal handling structures and functions are in iwl-fw.h.

Fix the separation between the files by moving file format
definitions to iwl-fw-file.h.

Also add a missing description for a kernel-doc member, and it's
not needed to include iwl-fw-file.h again from the code (thanks
to Liad Kaufman).

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Reviewed-by: Liad Kaufman <liad.kaufman@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>