wext: call cfg80211_change_iface() with wiphy lock held

This is needed now that all the driver callbacks are protected by
the wiphy lock rather than (just) the RTNL.

Fixes: a05829a7

 ("cfg80211: avoid holding the RTNL when calling the driver")
Reported-by:  <syzbot+d2d412349f88521938aa@syzkaller.appspotmail.com>
Link: https://lore.kernel.org/r/20210128183454.e81bc6789b4b.I5deb8b6bfdc8b4ea7696cb2447ee6c58c7ce9a4e@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>