Change the argument of snd_midi_process_event() to receive a const
snd_midi_op pointer and its callers respectively.  This allows further
optimizations.

There should be no functional changes by this patch.

Link: https://lore.kernel.org/r/20200103081714.9560-30-tiwai@suse.deSigned-off-by: Takashi Iwai <tiwai@suse.de>