mac80211: Recalculate channel-type on iface removal.

When a vif goes away, it could cause the super-chan
to be recalculated differently, so do that calculation
on iface removal.

Signed-off-by: Ben Greear <greearb@candelatech.com>
Acked-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>