iio: light: us5182d: Add missing error code assignment before test

It is likely that checking the result of 'pm_runtime_set_active' is
expected here.

Fixes: f0e5f57d ("iio: light: us8152d: Add power management support")

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>