iio: adc: ti-ads1015: fix comparator polarity setting

The comparator polarity field in config register is not correctly
initialized as per the interrupt trigger setting.  Because the bitfield
definision is wrong and bit shifting is missed.

Fixes: d9f39bab ("iio: adc: ti-ads1015: add threshold event support")
Cc: Daniel Baluta <daniel.baluta@gmail.com>
Cc: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>