ALSA: seq: midi: Fix -Wformat-truncation warning

The compile warnings with -Wformat-truncation appearing at
snd_seq_midisynth_probe() in seq_midi.c are false-positive; those must
fit within the given string size.

For suppressing the warning, replace snprintf() with scnprintf().
As stated in the above, truncation doesn't matter.

Link: https://lore.kernel.org/r/20230915082802.28684-3-tiwai@suse.de


Signed-off-by: Takashi Iwai <tiwai@suse.de>