ALSA: seq: potential out of bounds in do_control()

Smatch complains that "control" is user specifigy and needs to be
capped.  The call tree to understand this warning is quite long.

snd_seq_write()  <-- get the event from the user
  snd_seq_client_enqueue_event()
    snd_seq_deliver_event()
      deliver_to_subscribers()
        snd_seq_deliver_single_event()
          snd_opl3_oss_event_input()
            snd_midi_process_event()
              do_control()

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>