zd1211rw: detect stalled beacon interrupt faster

As USB_INT_ID_RETRY_FAILED can override USB_INT_ID_REGS, beacon interrupt
(CR_INTERRUPT) might be lost. Problem is that when device trigger CR_INTERRUPT
it disables HW interrupt. Now if USB_INT_ID_REGS with CR_INTERRUPT gets lost,
beacon interrupt stays disabled until beacon watchdog notices the stall. This
happen very often on heavy TX. Improve watchdog to trigger earlier, after three
missing beacon interrupts.

Signed-off-by: Jussi Kivilinna <[e-mail]>
Signed-off-by: John W. Linville <[e-mail]>

drivers/net/wireless/zd1211rw/zd_mac.c

···
 	spin_lock_irq(&mac->lock);
 	interval = mac->beacon.interval;
 	period = mac->beacon.period;
-	timeout = mac->beacon.last_update + msecs_to_jiffies(interval) + HZ;
+	timeout = mac->beacon.last_update +
+			msecs_to_jiffies(interval * 1024 / 1000) * 3;
 	spin_unlock_irq(&mac->lock);
 
 	if (interval > 0 && time_is_before_jiffies(timeout)) {