ASoC: dapm: Fix long_name pointer for DAPM muxes

DAPM muxes' long_name (widget name for not shared controls
and widget kcontrol name for shared controls) doesn't contain
the prefix name, hence the pointer used to create the kcontrol
shouldn't include the offset of prefix name. Otherwise, kcontrol
name will have garbage characters.

Additionally, for muxes which don't have a codec name_prefix
shouldn't use the widget name as a prefix to avoid name duplication,
as the widget name will be used as long_name parameter to create
their kcontrol.

Signed-off-by: Misael Lopez Cruz <misael.lopez@ti.com>
Signed-off-by: Simon Wilson <simonwilson@google.com>