iio: st_sensors: Describe function parameters in kernel-doc

Kernel documentation script complains that some of the function parameters
are not described:

.../common/st_sensors/st_sensors_trigger.c:29: warning: Function parameter or member 'indio_dev' not described in 'st_sensors_new_samples_available'
.../common/st_sensors/st_sensors_trigger.c:29: warning: Function parameter or member 'sdata' not described in 'st_sensors_new_samples_available'

Describe function parameters where it's appropriate.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>