nret is not set
Current code returned the number of channels as channel layout in that case,
and if nret is not set then unknown layouts are typically not supported.
Also use the common parsing code. Use a temporary workaround to parse an
unknown channel layout such as '13c', after a 1 year grace period only '13C'
will work.
Signed-off-by: Marton Balint <cus@passwd.hu>
