ath9k: Drop useless const on chanctx_event_delta() return type

drivers/net/wireless/ath/ath9k/channel.c:230: warning: type qualifiers ignored on function return type

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>