sudo apt-get update
sudo apt-get upgrade
sudo reboot

sudo dpkg -i linux-firmware_1.157.5_all.deb

sudo apt-get install build-essential linux-headers-generic
wget https://www.kernel.org/pub/linux/kernel/projects/backports/2015/03/13/backports-20150313.tar.xz
tar xvfJ backports-20150313.tar.xz
cd backports-20150313
make defconfig-ath10k
make
sudo make install

$ uname -r
4.2.0-16-generic

$ lspci
02:00.0 Network controller: Qualcomm Atheros QCA6174 802.11ac Wireless Network Adapter (rev 20)
$ lspci -n
02:00.0 0280: 168c:003e (rev 20)

[    2.360584] ath10k_pci 0000:02:00.0: Direct firmware load for ath10k/cal-pci-0000:02:00.0.bin failed with error -2
[    2.362523] ath10k_pci 0000:02:00.0: Direct firmware load for ath10k/QCA6174/hw2.1/board-pci-168c:003e:11ad:0804.bin failed with error -2
[    2.362527] ath10k_pci 0000:02:00.0: failed to load spec board file, falling back to generic: -2
[    2.362536] ath10k_pci 0000:02:00.0: Direct firmware load for ath10k/QCA6174/hw2.1/board.bin failed with error -2
[    2.362538] ath10k_pci 0000:02:00.0: failed to fetch generic board data: -2
[    2.362540] ath10k_pci 0000:02:00.0: failed to fetch board file: -2
[    2.362541] ath10k_pci 0000:02:00.0: could not fetch firmware files (-2)
[    2.362543] ath10k_pci 0000:02:00.0: could not probe fw (-2)

/lib/firmware/ath10k/QCA6174/hw2.1

Direct firmware load for ath10k/cal-pci-0000:03:00.0.bin failed with error -2
invalid firmware magic
Direct firmware load for ath10k/QCA6174/hw2.1/firmware-3.bin failed with error -2
could not fetch firmware file 'ath10k/QCA6174/hw2.1/firmware-3.bin': -2

sudo rmmod ath10k_pci
sudo modprobe -v ath10k_pci