ASoC: wm_adsp: wm_adsp_control_add() error: uninitialized symbol 'ret'

ASoC: wm_adsp: wm_adsp_control_add() error: uninitialized symbol 'ret'

This patch fixes the static analysis warning as it is correctly
indicating a possible code path, it cannot know that for the affected
firmware versions subname would always be NULL.

Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Simon Trimmer <simont@opensource.cirrus.com>
Acked-by: Charles Keepax <ckeepax@opensource.cirrus.com>
Link: https://lore.kernel.org/r/20211115120154.56782-1-simont@opensource.cirrus.com
Signed-off-by: Mark Brown <broonie@kernel.org>