nl80211: Emit a SET_INTERFACE on iftype change

Let userspace learn about iftype changes by sending a notification
when handling the NL80211_CMD_SET_INTERFACE command.  There seems
to be no other place where the iftype can change: nl80211_set_interface
is the only caller of cfg80211_change_iface which is the only caller of
ops->change_virtual_intf.

Signed-off-by: Andrew Zaborowski <andrew.zaborowski@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>