staging: iio: tsl2x7x: check return value from tsl2x7x_invoke_change()

The return value from tsl2x7x_invoke_change() was not checked in most
places in the driver. This patch adds the proper error checks. The
return values inside tsl2x7x_invoke_change() are now checked by
this patch as well.

Previously, if there was an error turning the chip back on, then the
driver would attempt to turn the chip off and incorrectly return
success. The code to power off the chip is removed by this patch
since we should fail fast.

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