ath6kl: fix warning for using 0 as NULL

Fixes the following sparse warning:

drivers/net/wireless/ath/ath6kl/sdio.c:716:55: warning:
 Using plain integer as NULL pointer

Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>