iio: adc: ad7280a: Fix wrong variable used when setting thresholds.

Name of variable change missed in refactoring patch.

Fixes: 112bf4aa ("staging:iio:adc:ad7280a: Switch to standard event control")
Reported-by: Colin Ian King <colin.i.king@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: Marcelo Schmitt <marcelo.schmitt1@gmail.com>
Reviewed-by: Marcelo Schmitt <marcelo.schmitt1@gmail.com>
Link: https://lore.kernel.org/r/20220226175604.662422-1-jic23@kernel.org