virt_wifi: fix deadlock on RTNL

Fix a regression where everything in virt_wifi would just hang. This
happened due to overlapping changes between commit a05829a7
("cfg80211: avoid holding the RTNL when calling the driver") which
had originally needed to change the locking, but then I introduced
commit 2fe8ef10 ("cfg80211: change netdev registration/unregistration
semantics") instead. virt_wifi somehow fell through the cracks when
I undid all the previous locking changes. Fix it now.

Fixes: a05829a7 ("cfg80211: avoid holding the RTNL when calling the driver")
Reported-by:  <syzbot+3d2d5e6cc3fb15c6a0fd@syzkaller.appspotmail.com>
Link: https://lore.kernel.org/r/20210127215941.2d6a97b09784.I4f1fac32f67045171be50931f44d77e150911bee@changeid


Signed-off-by: Johannes Berg <johannes.berg@intel.com>