iio: accel: adxl345: simplify measure enable

Simplify the function to enable or disable measurement. Replace the
separate decision logic and call to regmap_update_bits() by a single
call to regmap_assign_bits() taking a boolean argument directly.

This is a refactoring change and should not impact functionality.

Signed-off-by: Lothar Rubusch <l.rubusch@gmail.com>
Link: https://patch.msgid.link/20250610215933.84795-4-l.rubusch@gmail.com


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