staging: iio: ade7854-spi: Fix return value

ade7854_probe can fail. Return the value obtained from it
instead of 0 (success).

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Cc: Barry Song <21cnbao@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>