staging: iio: tsl2x7x: add common function for reading chip status

There were three places where the same chunk of code was used to read
the chip status. This patch creates a common function
tsl2x7x_read_status() to reduce duplicate code. This patch also corrects
tsl2x7x_event_handler() to properly check for an error after reading the
chip status.

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