ath5k: Use SWI to trigger calibration

* Get rid of calibration timer, instead use a software interrupt
  to schedule the calibration tasklet.

 a) We don't need a timer for this, there is no need for accuracy
   even with round_jiffies i think this is a waste of resources.
   Also we don't need to run calibration if we are idle (no
   interrupts).

 b) When we add ANI support we 'll just extend the poll function
   and calibration tasklet and handle all periodic phy calibration
   on one place (much cleaner).

 c) Having calibration on a tasklet is better since during calibration
   we can't transmit or receive (antennas are detached to measure
   noise floor), previously calibration could run in parallel with
   tx/rx and interfere (packet loss).

 v2: kill tasklet on stop_hw, stop/wake queues
 v3: use time_is_before_eq_jiffies to compare timestamp with current
     time

Signed-off-by: Nick Kossifidis <mickflemm@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

drivers/net/wireless/ath/ath5k/base.h

@@ -177,6 +177,8 @@ struct ath5k_softc {
 
 	struct ath5k_rfkill	rf_kill;
 
+	struct tasklet_struct	calib;		/* calibration tasklet */
+
 	spinlock_t		block;		/* protects beacon */
 	struct tasklet_struct	beacontq;	/* beacon intr tasklet */
 	struct ath5k_buf	*bbuf;		/* beacon buffer */
@@ -187,7 +189,6 @@ struct ath5k_softc {
 	unsigned int		nexttbtt;	/* next beacon time in TU */
 	struct ath5k_txq	*cabq;		/* content after beacon */
 
-	struct timer_list	calib_tim;	/* calibration timer */
 	int 			power_level;	/* Requested tx power in dbm */
 	bool			assoc;		/* assocate state */
 	bool			enable_beacon;	/* true if beacons are on */