ALSA: ump: Fix the discard error code from snd_ump_legacy_open()

snd_ump_legacy_open() didn't return the error code properly even if it
couldn't open.  Fix it.

Fixes: 0b5288f5

 ("ALSA: ump: Add legacy raw MIDI support")
Cc: <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20240220150843.28630-1-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>