ALSA: seq: oss: Fix unbalanced use lock for synth MIDI device

ALSA: seq: oss: Fix unbalanced use lock for synth MIDI device

commit f5e94b4c6ebdabe0f602d796e0430180927521a0 upstream.

When get_synthdev() is called for a MIDI device, it returns the fixed
midi_synth_dev without the use refcounting.  OTOH, the caller is
supposed to unreference unconditionally after the usage, so this would
lead to unbalanced refcount.

This patch corrects the behavior and keep up the refcount balance also
for the MIDI synth device.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>