nl80211: call cfg80211_dev_rename() under RTNL

This is required, and we have an assertion, move the RTNL
unlock down to cover cfg80211_dev_rename().

Fixes: a05829a7 ("cfg80211: avoid holding the RTNL when calling the driver")
Reported-by:  <syzbot+ed107c5fa3e21cdcd86e@syzkaller.appspotmail.com>
Link: https://lore.kernel.org/r/20210128183454.16a4ea036740.I2beeb391dc322d6f1df3ee2612e714e0a7817397@changeid


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