Fix value of "scale" stored in XML without quotes

Signed-off-by: Paul Cercueil <paul.cercueil@analog.com>
Local backend: Fix channel indexes not properly initialized

Signed-off-by: Paul Cercueil <paul.cercueil@analog.com>
Xml backend: Fix channel indexes incorrectly initialized to 0

They should be initialized to a negative value instead, as 0 is a valid
channel index.

Signed-off-by: Paul Cercueil <paul.cercueil@analog.com>