ath: fix SPDX tags

Commit ec8f24b7

 ("treewide: Add SPDX license identifier -
Makefile/Kconfig") marked various Makefiles and Kconfig files within ath
directories as GPL-2.0. But these modules and drivers are actually ISC:

* ath
* ar5523
* ath10k
* ath5k
* ath6kl
* ath9k
* wcn36xx
* wil6210

Fix SPDX tags accordingly.

Signed-off-by: Kalle Valo <kvalo@codeaurora.org>