ath10k: destroy sdio workqueue while remove sdio module

The workqueue need to flush and destory while remove sdio module,
otherwise it will have thread which is not destory after remove
sdio modules.

Tested with QCA6174 SDIO with firmware
WLAN.RMH.4.4.1-00007-QCARMSWP-1.

Signed-off-by: Wen Gong <wgong@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>