staging: iio: tsl2x7x: refactor {read,write}_event_value to allow handling multiple iio_event_infos

tsl2x7x_read_thresh() and tsl2x7x_write_thresh() currently assumes
that IIO_EV_INFO_VALUE is the only iio_event_info that will be
passed in.  This patch refactors these two functions so that
additional iio_event_infos can be passed in. The functions are
renamed from tsl2x7x_{read,write}_thresh() to
tsl2x7x_{read,write}_event_value(). This patch also adds the
missing return value check to tsl2x7x_invoke_change() since this
was previously missing. This patch is in preparation for moving
the in_intensity0_thresh_period and in_proximity0_thresh_period sysfs
attributes to be created by iio_event_spec.

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