iio: accel: adxl345: make data struct variable irq function local

Remove variable irq from the struct state and make it a function local
variable, because it is not necessary to be kept struct-wise.

This is a refactoring change and should not impact functionality.

Signed-off-by: Lothar Rubusch <l.rubusch@gmail.com>
Link: https://patch.msgid.link/20250610215933.84795-3-l.rubusch@gmail.com


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