HID: prodikeys: constify snd_rawmidi_ops structures

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

Done with the help of Coccinelle.

Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>