ath11k: remove unneeded flush_workqueue

All work currently pending will be done first by calling destroy_workqueue,
so there is no need to flush it explicitly.

Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: Lv Ruyi (CGEL ZTE) <lv.ruyi@zte.com.cn>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://lore.kernel.org/r/20220301013246.2052570-1-lv.ruyi@zte.com.cn