mac80211: set short_slot for 6 GHz band

Set short slot also for 6 GHz band, just like 5 GHz.

Signed-off-by: Tova Mussai <tova.mussai@intel.com>
Link: https://lore.kernel.org/r/20200528213443.75f38e6f5efd.I272fbae402b03123f04e9ae69204eeab960c70cd@changeid


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