ath10k: cancel coverage class work during stop and restart

It seems set_coverage_class_work is not cancelled anywhere,
though I could not find a crash/warning  with this existing
design, its safer to cancel it during stop() and also before
restarting the hardware.

Signed-off-by: Mohammed Shafi Shajakhan <mohammed@qti.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>