iio: accel: adxl345: simplify reading the FIFO

Bulk FIFO reading can be streamlined by eliminating redundant variables and
simplifying the process of reading x-, y-, and z-axis measurement sets.

This is a refactoring change with no expected impact on functionality.

Signed-off-by: Lothar Rubusch <l.rubusch@gmail.com>
Link: https://patch.msgid.link/20250702230315.19297-3-l.rubusch@gmail.com


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