iio: adc: stm32: Fix missing return in booster error path

iio: adc: stm32: Fix missing return in booster error path

Conversion of error paths to dev_err_probe() dropped one return
statement.  The driver will continue to probe if getting booster
regulator fails.

Fixes: 84839a16f540 ("iio: adc: stm32: Simplify with dev_err_probe()")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Link: https://lore.kernel.org/r/1599727927-7776-1-git-send-email-krzk@kernel.org
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>