ath6kl: fix crash when interface is closed but scan is ongoing

When ath6kl module was removed while a scan was ongoing the driver would
crash in ath6kl_cfg80211_scan_complete_event().

Fix the function not to iterate nodes when the scan is aborted. The nodes
are already freed when the module is being unloaded. This patch removes the
null check entirely as the wmi structure is not accessed anymore during
module unload.

Also fix a bug where the status was checked as a bitfield with '&' operator.
But it's not a bitfield, just a regular error code.

This is a port of my patch from ath6kl staging with the same title.

Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>