WiFi driver installation fails - SSL error:02001002:system library:fopen:No such file or directory: bss_file.c:175

This error, upon loading the driver, shows the result of a faulty make and sudo make install:

sudo modprobe ath10k_pci modprobe: ERROR: could not insert 'ath10k_pci': Required key not available 

Let's uninstall the driver you built from backports. It is not needed as it already exists in Ubuntu 16.04:

cd ~/backports-4.4.2-1
sudo make uninstall

Reboot and check to see if the default driver is loaded:

lsmod | grep ath

If so, the message log should show us what firmware is required, if not the firmware you already installed. Check:

dmesg | grep ath

If, in fact, firmware is missing, I will edit this answer to explain how to install it.