mac80211: dynamic short slot time for MBSSs

The standard mandates mesh STAs to set the ERP Short Slot
Time capability info bit in beacons to 0. Even though this
is their way of disallowing short slot time for mesh STAs,
there should be no harm in enabling it if we determine all
STAs in the current MBSS support ERP rates.

Increases throughput about 20% for legacy rates when
enabled.

Signed-off-by: Thomas Pedersen <thomas@cozybit.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>