ALSA: line6: Fix memory leak at line6_init_pcm() error path

I forgot to release the allocated object at the early error path in
line6_init_pcm().  For addressing it, slightly shuffle the code so
that the PCM destructor (pcm->private_free) is assigned properly
before all error paths.

Fixes: 34501219 ("ALSA: line6: Fix write on zero-sized buffer")
Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>