staging: iio: tsl2x7x: use direct returns

This patch changes the functions tsl2x7x_read_event_value() and
tsl2x7x_read_raw() to use direct returns to simplify the code.

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