iio: imu: st_lsm6dsx: add read_fifo callback to fifo_ops

Remove static qualifier from st_lsm6dsx_read_fifo definition and
introduce read_fifo function pointer in fifo_ops data structure
in order to run the proper read_fifo routine since other compliant
devices will use a different FIFO queueing scheme.

Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>