ALSA: rawmidi: Add ioctl callback to snd_rawmidi_global_ops

A new callback, ioctl, is added to snd_rawmidi_global_ops for allowing
the driver to deal with the own ioctls.  This is another preparation
patch for the upcoming UMP support.

Reviewed-by: Jaroslav Kysela <perex@perex.cz>
Link: https://lore.kernel.org/r/20230523075358.9672-3-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>