ASoC: tpa6130a2: Get rid of compile warning from tpa6130a2_power

Patch "ASoC: tpa6130a2: Fix unbalanced regulator disables" introduced a
compiler warning "‘ret’ may be used uninitialized in this function".
Initialize ret to zero to get rid of it and making sure that the function
does not return any random error code when the code is falling through.

Signed-off-by: Jarkko Nikula <jhnikula@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>