ath9k: Fix typo in function name

Typo "destoy" made me wonder if correct patch is wrong; fix it. No
functional change.

Signed-off-by: Pavel Machek (CIP) <pavel@denx.de>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200724083910.GA31930@amd