usb: gadget: f_midi: constify snd_rawmidi_ops structures

These snd_rawmidi_ops structures are only passed as the third
argument of snd_rawmidi_set_ops.  This argument is const, so the
snd_rawmidi_ops structures can be const too.

Done with the help of Coccinelle.

Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>