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>

drivers/net/wireless/iwlegacy/3945.h

@@ -150,10 +150,6 @@ struct il3945_frame {
 	struct list_head list;
 };
 
-#define SEQ_TO_SN(seq) (((seq) & IEEE80211_SCTL_SEQ) >> 4)
-#define SN_TO_SEQ(ssn) (((ssn) << 4) & IEEE80211_SCTL_SEQ)
-#define MAX_SN ((IEEE80211_SCTL_SEQ) >> 4)
-
 #define SUP_RATE_11A_MAX_NUM_CHANNELS  8
 #define SUP_RATE_11B_MAX_NUM_CHANNELS  4
 #define SUP_RATE_11G_MAX_NUM_CHANNELS  12