ath10k: fix ath10k_debug_start() locking

ath10k_debug_start() was not called with conf_mutex, fix that. Also there was a
deadlock in ath10k_debug_stop(), rename it to ath10k_debug_destroy() and call
it only when the device is destroyed.

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