iio: accel: adxl345: replace magic numbers by unit expressions

Replace absolute numbers by their expressions from units.h to avoid
using magic numbers. Use uniform expressions to clarify their usage.

This is a refactoring change and should not impact functionality.

Signed-off-by: Lothar Rubusch <l.rubusch@gmail.com>
Link: https://patch.msgid.link/20250610215933.84795-7-l.rubusch@gmail.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>