mac80211: mesh: don't use global channel type

Using local->_oper_channel_type in the mesh code is
completely wrong as this value is the combination
of the various interface channel types and can be
a different value from the mesh interface in case
there are multiple virtual interfaces.

Use sdata->vif.bss_conf.channel_type instead as it
tracks the per-vif channel type.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>