ath10k: switch to use SPDX license identifiers

Use SPDX identifiers everywhere in ath10k.

Makefile was incorrectly marked in commit b2441318 ("License cleanup: add
SPDX GPL-2.0 license identifier to files with no license"), fix that as well.

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