pcm: route: Correctly close slave pcm when no matching chmap is found

This patch fixes a bug where the slave pcm was not correctly closed
on some error conditions, such as not finding a matching chmap.

Reported-by: Raymond Yau <superquad.vortex2@gmail.com>
Signed-off-by: David Henningsson <david.henningsson@canonical.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>