ASoC: Intel: Skylake: Return negative error code

skl_tplg_add_pipe() returned EEXIST instead of negative EEXIST, so fix that
and handle the return value as well.

Signed-off-by: Guneshwor Singh <guneshwor.o.singh@intel.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>
Reviewed-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Mark Brown <broonie@kernel.org>