ath10k: switch to use SPDX license identifiers
Use SPDX identifiers everywhere in ath10k.
Makefile was incorrectly marked in commit b24413180f56 ("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>