staging: iio: tsl2x7x: add common function for writing to the control register

staging: iio: tsl2x7x: add common function for writing to the control register

There were four places where the same chunk of code was used to write
to the control register. This patch creates a common function
tsl2x7x_write_control_reg() to reduce duplicate code.

The function tsl2x7x_chip_off() did not correctly return an error
code so this patch also corrects that issue.

Signed-off-by: Brian Masney <masneyb@onstation.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>