iwlwifi: use proper short slot/preamble settings

The short preamble setting might change on the
fly, and then we already use the right mac80211
variable. However, in other places we don't,
which is especially wrong in the AP code since
in that case the assoc_capability is invalid.
Also, the IBSS special case is not needed since
"use_short_slot" will be properly cleared, but
the "assoc_capability" might be invalid (which
must be the reason for the special case).

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>