wireless: move sequence number arithmetic to ieee80211.h

Move the sequence number arithmetic code from mac80211 to
ieee80211.h so others can use it. Also rename the functions
from _seq to _sn, they operate on the sequence number, not
the sequence_control field.

Also move macros to convert the sequence control to/from
the sequence number value from various drivers.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
diff --git a/drivers/net/wireless/iwlwifi/mvm/tx.c b/drivers/net/wireless/iwlwifi/mvm/tx.c
index 6b67ce3..56df249 100644
--- a/drivers/net/wireless/iwlwifi/mvm/tx.c
+++ b/drivers/net/wireless/iwlwifi/mvm/tx.c
@@ -641,7 +641,7 @@
 		next_reclaimed = ssn;
 	} else {
 		/* The next packet to be reclaimed is the one after this one */
-		next_reclaimed = SEQ_TO_SN(seq_ctl + 0x10);
+		next_reclaimed = IEEE80211_SEQ_TO_SN(seq_ctl + 0x10);
 	}
 
 	IWL_DEBUG_TX_REPLY(mvm,