iwlwifi: change struct iwl_fw

Change iwl_fw struct to hold an array of fw_img instead of
three separated instances.

Change fw_img to hold an array of fw_desc instead of two
separate descriptors for instructions and data.

Change load_given_ucode, load_section, verification functions
etc. to support this structure.

Signed-off-by: David Spinadel <david.spinadel@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>