wl18xx: make sure fw_status->priv exists before deref

In some corner cases with specific timings, we might
try dequeueing tx before we got information about
the link status (e.g. due to recovery during tx).

Instead of NULL dereference, assume all
the links in this case have low priorities.

Signed-off-by: Eliad Peller <eliad@wizery.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>