mac80211: always call wireless_set_data (FS#3784)

mac80211: always call wireless_set_data  (FS#3784)

When wifi is turned off, drv_mac80211_teardown sometimes fails (silently)
because the device to be torn down is not defined.

This situation arises if drv_mac80211_setup was called twice when
wifi was turned on.

This commit ensures that the device to be torn down is always defined
in drv_mac80211_teardown.

Steps to reproduce:

1) Use /sbin/wifi to turn on wifi.
   uci set wireless.@wifi-iface[0].disabled=0
   uci set wireless.@wifi-device[0].disabled=0
   uci commit
   wifi

2) Use /sbin/wifi to turn off wifi.
   uci set wireless.@wifi-device[0].disabled=1
   uci commit
   wifi

3) Observe that wifi is still up.

branches affected: trunk, 21.02

Signed-off-by: Bob Cantor <coxede6557@w3boats.com>
(cherry-picked from commit d515f6b6cde357bf480d32a7387f07ea40e85e52)