iio: accel: adxl345: simplify interrupt mapping

iio: accel: adxl345: simplify interrupt mapping

Refactor the sensor interrupt mapping by utilizing regmap_assign_bits(),
which accepts a boolean value directly. Introduce a helper function to
streamline the identification of the configured interrupt line pin. Also,
use identifiers from units.h to represent the full 8-bit register when
setting bits.

This is a purely refactoring change and does not affect functionality.

Signed-off-by: Lothar Rubusch <l.rubusch@gmail.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://patch.msgid.link/20250702230315.19297-2-l.rubusch@gmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>