ALSA: usb: Constify snd_rawmidi_ops

Now snd_rawmidi_ops is maintained as a const pointer in snd_rawmidi,
we can constify the definitions.

Reviewed-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>