ath9k: Use tasklet_disable_in_atomic()

All callers of ath9k_beacon_ensure_primary_slot() are preemptible /
acquire a mutex except for this callchain:

  spin_lock_bh(&sc->sc_pcu_lock);
  ath_complete_reset()
  -> ath9k_calculate_summary_state()
     -> ath9k_beacon_ensure_primary_slot()

It's unclear how that can be distangled, so use tasklet_disable_in_atomic()
for now. This allows tasklet_disable() to become sleepable once the
remaining atomic users are cleaned up.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Kalle Valo <kvalo@codeaurora.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20210309084242.313899703@linutronix.de