iwlwifi: mvm: prepare for station count change

In a000 devices we will support up to 32 stations.
The max station define is used also for invalid station marking
which makes finding usages of actual maximum station pretty hard
to sort through - change it to be a different define in order
to make future changes easier.
Use also ARRAY_SIZE intead of define when possible.
Do not move yet to 32 stations until firmware do it though.

Signed-off-by: Sara Sharon <sara.sharon@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
diff --git a/drivers/net/wireless/intel/iwlwifi/mvm/fw-api-mac.h b/drivers/net/wireless/intel/iwlwifi/mvm/fw-api-mac.h
index 480a54a..d3cdd88 100644
--- a/drivers/net/wireless/intel/iwlwifi/mvm/fw-api-mac.h
+++ b/drivers/net/wireless/intel/iwlwifi/mvm/fw-api-mac.h
@@ -73,7 +73,9 @@
 #define NUM_MAC_INDEX		(NUM_MAC_INDEX_DRIVER + 1)
 #define NUM_MAC_INDEX_CDB	(NUM_MAC_INDEX_DRIVER + 2)
 
-#define IWL_MVM_STATION_COUNT	16
+#define IWL_MVM_STATION_COUNT		16
+#define IWL_MVM_INVALID_STA		0xFF
+
 #define IWL_MVM_TDLS_STA_COUNT	4
 
 enum iwl_ac {