soc: mediatek: pm-domains: Fix missing error code in scpsys_add_subdomain()

Adding one power domain in scpsys_add_subdomain is missing to assign an
error code when it fails. Fix that assigning an error code to 'ret',
this also fixes the follwowing smatch warning.

  drivers/soc/mediatek/mtk-pm-domains.c:492 scpsys_add_subdomain() warn: missing error code 'ret'

Fixes: dd650302 ("soc: mediatek: pm-domains: Don't print an error if child domain is deferred")
Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Link: https://lore.kernel.org/r/20210303091054.796975-1-enric.balletbo@collabora.com


Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>