ALSA: rawmidi: fix the get next midi device ioctl

If we pass in a device which is higher than SNDRV_RAWMIDI_DEVICES then
the "next device" should be -1.  This function just returns device + 1.

But the main thing is that "device + 1" can lead to a (harmless) integer
overflow and that annoys static analysis tools.

[fix the case for device == SNDRV_RAWMIDI_DEVICE by tiwai]

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>