ALSA: seq: Fix time account regression

The recent rewrite of the sequencer time accounting using timespec64
in the commit [3915bf294652: ALSA: seq_timer: use monotonic times
internally] introduced a bad regression.  Namely, the time reported
back doesn't increase but goes back and forth.

The culprit was obvious: the delta is stored to the result (cur_time =
delta), instead of adding the delta (cur_time += delta)!

Let's fix it.

Fixes: 3915bf294652 ('ALSA: seq_timer: use monotonic times internally')
Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=177571
Reported-by: Yves Guillemot <yc.guillemot@wanadoo.fr>
Cc: <stable@vger.kernel.org> # v4.8+
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/core/seq/seq_timer.c

@@ -448,8 +448,8 @@ snd_seq_real_time_t snd_seq_timer_get_cur_time(struct snd_seq_timer *tmr)
 
 		ktime_get_ts64(&tm);
 		tm = timespec64_sub(tm, tmr->last_update);
-		cur_time.tv_nsec = tm.tv_nsec;
-		cur_time.tv_sec = tm.tv_sec;
+		cur_time.tv_nsec += tm.tv_nsec;
+		cur_time.tv_sec += tm.tv_sec;
 		snd_seq_sanity_real_time(&cur_time);
 	}
 	spin_unlock_irqrestore(&tmr->lock, flags);