mac80211: fix handling of 4-address-mode in ieee80211_change_iface

A misplaced interface type check bails out too early if the interface
is not in monitor mode. This patch moves it to the right place, so that
it only covers changes to the monitor flags.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Cc: stable@kernel.org
Signed-off-by: John W. Linville <linville@tuxdriver.com>