iwlwifi: Renames struct fw_image_desc to struct fw_desc

This patche shortens the name of struct fw_image_desc to be struct
fw_desc.

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Zhu Yi <yi.zhu@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>