staging: iio: tsl2x7x: remove redundant power_state sysfs attribute

The TSL2X7X driver has a custom power_state sysfs attribute. Remove this
attribute since the runtime power management code provides a sysfs
attribute to control the power state of the device.

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