iio: accel: st_accel: Move platform data from header to C file

iio: accel: st_accel: Move platform data from header to C file

Platform data is solely used by one file. Don't share it with others.

While at it, drop unneeded anymore __maybe_unused and fix kernel doc
to avoid warning:

  st_accel_core.c:1079: error: Cannot parse struct or union!

by converting to a simple comment. It is described at the declaration.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20210414195454.84183-1-andriy.shevchenko@linux.intel.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>