iio: imu: st_lsm6dsx: add error logs to st_lsm6dsx_read_fifo()

Add debug info to error conditions in st_lsm6dsx_read_fifo routine

Suggested-by: Jorge Ramirez-Ortiz <jramirez@baylibre.com>
Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>