staging: iio: tsl2x7x: turn chip off if IIO device registration fails

This patch turns the chip off if IIO device registration fails so that
the error handling mirrors the device remove to make review easier in
preparation for moving this driver out of staging.

This patch also adds a missing error check in the call to
tsl2x7x_chip_on() in tsl2x7x_probe().

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