ALSA: rawmidi: Fix NULL dereference at proc read

At the implementation of the optional proc fs in rawmidi, I forgot
that rmidi->ops itself is optional and can be NULL.
Add the proper NULL check for avoiding the Oops.

Fixes: fa030f66

 ("ALSA: ump: Additional proc output")
Reported-and-tested-by: Mark Hills <mark@xwax.org>
Closes: https://lore.kernel.org/r/ef9118c3-a2eb-d0ff-1efa-cc5fb6416bde@xwax.org
Cc: <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20230916060725.11726-1-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>