You can do either way: Upgrade the FUS first or upgrade the firmware first.
The main thing to consider is after doing either one, reboot the controller and then do the next.
Here you go for latest FUS release and download and upgrade images.
Please refer to the below links :
•After you install the FUS image for a Cisco Flex 7500 Controller platform, the RAID firmware is also upgraded. During the installation process, the console messages displayed do indicate upgrade of the RAID firmware. However, it is not possible to verify the RAID firmware upgrade either by entering a command or viewing the bootlog.