install ath9k firmware on both $(prefix) and $(prefix)/ath9k_htc

install ath9k firmware on both $(prefix) and $(prefix)/ath9k_htc

Having ath9k_htc fw files installed at $(prefix)/ makes it work, but not having them in $(prefix)/ath9k_htc
causes the following:

[    8.952592] usb 3-2: ath9k_htc: Firmware ath9k_htc/htc_9271-1.4.0.fw requested
[    8.952652] usb 3-2: Direct firmware load for ath9k_htc/htc_9271-1.4.0.fw failed with error -2
[    8.952655] usb 3-2: ath9k_htc: Firmware htc_9271.fw requested

Although the firmware works, it throws that (benign) error, that's why linux-firmware installs it in both,
/lib/firmware and /lib/firmware/ath9k_htc, and this patch fixes this by creating a symlink.

kernel log after the patch:

[    3.929726] Workqueue: events request_firmware_work_func
[    3.929769]  ? _request_firmware+0x423/0xcc0
[    3.929772]  request_firmware_work_func+0x47/0x90
[    5.408573] usb 3-2: ath9k_htc: Firmware ath9k_htc/htc_9271-1.4.0.fw requested

Signed-off-by: David P <megver83@parabola.nu>