ath10k: document switch case fall-through in __ath10k_scan_finish

Add a comment for indicating that the ATH10K_SCAN_RUNNING case falls
through to the ATH10K_SCAN_ABORTING case in __ath10k_scan_finish.  This
will document that the lack of a break is intentional.

Coverity: CID 1260017

Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>