staging: iio: tsl2583: Use devm_iio_device_alloc

devm_iio_device_alloc makes the code simple. While at it also
fixed an uninitialized return with -EINVAL.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Cc: J. August Brenner <jbrenner@taosinc.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>