mac80211: adjust reserved chan_ctx when assigned to vif

When a vif starts using a reserved channel context (during CSA, for example)
the required chandef was recalculated, however it was never applied.
This could result in using chanctx with narrower width than actually
required. Fix this by calling ieee80211_change_chanctx with the recalculated
chandef. This both changes the chanctx's width and recalcs min_def.

Signed-off-by: Andrei Otcheretianski <andrei.otcheretianski@intel.com>
Reviewed-by: Luciano Coelho <luciano.coelho@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>