staging:iio:ad7150: fix threshold mode config bit

According to the AD7150 configuration register description, bit 7 assumes
value 1 when the threshold mode is fixed and 0 when it is adaptive,
however, the operation that identifies this mode was considering the
opposite values.

This patch renames the boolean variable to describe it correctly and
properly replaces it in the places where it is used.

Fixes: 531efd6a

 ("staging:iio:adc:ad7150: chan_spec conv + i2c_smbus commands + drop unused poweroff timeout control.")
Signed-off-by: Melissa Wen <melissa.srw@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>