iio: light: ltr501: Powerdown device on error

Power down device when an error occurs  in order to avoid wasting
power. Move powerdown function up to be seen by the new call and
align parameters for the ltr501_write_contr() call.

Signed-off-by: Cristina Opriceana <cristina.opriceana@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>