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-commands.h b/drivers/net/wireless/iwlwifi/iwl-commands.h
index abe2479..935b19e 100644
--- a/drivers/net/wireless/iwlwifi/iwl-commands.h
+++ b/drivers/net/wireless/iwlwifi/iwl-commands.h
@@ -4370,6 +4370,11 @@
  * REPLY_WIPAN_PARAMS = 0xb2 (Commands and Notification)
  */
 
+/*
+ * Minimum slot time in TU
+ */
+#define IWL_MIN_SLOT_TIME	20
+
 /**
  * struct iwl_wipan_slot
  * @width: Time in TU