ath10k: hif: make send_complete_check op optional

That way we don't need to have an empty function in sdio.c.

No functional changes, compile tested only.

Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/1587037859-28873-5-git-send-email-kvalo@codeaurora.org