Revert "SW: drop firmware-ath9k-htc from GRMLBASE"

This reverts commit ebb166f.

It's now back in Debian/testing, see
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=997767#28

Thanks, Chris for reporting its progress to us

Closes: #107

etc/grml/fai/config/package_config/GRMLBASE

@@ -62,6 +62,7 @@ xz-utils
 # firmware packages
 atmel-firmware
 bluez-firmware
+firmware-ath9k-htc
 firmware-atheros
 firmware-bnx2
 firmware-bnx2x