ath6kl: wmi: fix an error code in ath6kl_wmi_sync_point()

This error path is unlikely because of it checked for NULL and
returned -ENOMEM earlier in the function.  But it should return
an error code here as well if we ever do hit it because of a
race condition or something.

Fixes: bdcd8170 ("Add ath6kl cleaned up driver")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20210813113438.GB30697@kili