cppcheck warning:

sound/soc/sh/rcar/ctu.c:212:6: style: Variable 'ret' is reassigned a
value before the old one has been used. [redundantAssignment]
 ret = rsnd_kctrl_new_m(mod, io, rtd, "CTU SV0",
     ^
sound/soc/sh/rcar/ctu.c:205:6: note: ret is assigned
 ret = rsnd_kctrl_new_m(mod, io, rtd, "CTU Pass",
     ^
sound/soc/sh/rcar/ctu.c:212:6: note: ret is overwritten
 ret = rsnd_kctrl_new_m(mod, io, rtd, "CTU SV0",
     ^

All the kcontrol creations are checked for errors, except for one. Add
the missing error check.
Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Link: https://lore.kernel.org/r/20210219231635.5749-6-pierre-louis.bossart@linux.intel.comSigned-off-by: Mark Brown <broonie@kernel.org>