staging: iio: tsl2x7x: use either direction for IIO_EV_INFO_{ENABLE,PERIOD}

The events IIO_EV_INFO_VALUE and IIO_EV_INFO_ENABLE currently have a
falling and rising direction configured. There does not need to be a
separate distinction so this patch changes these to use the
either direction. Directory listing of event sysfs attributes for a
TSL2772 with this patch applied:

in_intensity0_thresh_either_en
in_intensity0_thresh_either_period
in_intensity0_thresh_falling_value
in_intensity0_thresh_rising_value
in_proximity0_thresh_either_en
in_proximity0_thresh_either_period
in_proximity0_thresh_falling_value
in_proximity0_thresh_rising_value

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