iio: st_sensors: fix errorcheck for regulators

We were checking the return code of vdd when we should be checking
vdd_io. My mistake, mea culpa.

Cc: Giuseppe BARBA <giuseppe.barba@st.com>
Reported-by: Giuseppe BARBA <giuseppe.barba@st.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>