wifi: carl9170: do not ping device which has failed to load firmware

In the Linux kernel, the following vulnerability has been resolved:

wifi: carl9170: do not ping device which has failed to load firmware

Syzkaller reports [1, 2] crashes caused by an attempts to ping

the device which has failed to load firmware. Since such a device

doesn't pass 'ieee80211_register_hw()', an internal workqueue

managed by 'ieee80211_queue_work()' is not yet created and an

attempt to queue work on it causes null-ptr-deref.

[1] https://syzkaller.appspot.com/bug?extid=9a4aec827829942045ff

[2] https://syzkaller.appspot.com/bug?extid=0d8afba53e8fb2633217