ALSA: emu10k1: Fix deadlock in synth voice lookup

The emu10k1 voice allocator takes voice_lock spinlock.  When there is
no empty stream available, it tries to release a voice used by synth,
and calls get_synth_voice.  The callback function,
snd_emu10k1_synth_get_voice(), however, also takes the voice_lock,
thus it deadlocks.

The fix is simply removing the voice_lock holds in
snd_emu10k1_synth_get_voice(), as this is always called in the
spinlock context.

Reported-and-tested-by: Arthur Marsh <arthur.marsh@internode.on.net>
Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>