iwlwifi: replace minimum slot time constant

There are a number of places where the minimum
slot time is hardcoded to 20 TU, add a new
constant for that and use it everywhere.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
diff --git a/drivers/net/wireless/iwlwifi/iwl-agn.c b/drivers/net/wireless/iwlwifi/iwl-agn.c
index 51e5ea4..02771ef 100644
--- a/drivers/net/wireless/iwlwifi/iwl-agn.c
+++ b/drivers/net/wireless/iwlwifi/iwl-agn.c
@@ -3792,7 +3792,7 @@
 	queue_delayed_work(priv->workqueue, &priv->_agn.hw_roc_work,
 			   msecs_to_jiffies(duration + 20));
 
-	msleep(20);
+	msleep(IWL_MIN_SLOT_TIME); /* TU is almost ms */
 	ieee80211_ready_on_channel(priv->hw);
 
  out: