ALSA: seq: Fix time account regression

The recent rewrite of the sequencer time accounting using timespec64
in the commit [3915bf29: 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: 3915bf29 ('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>