spi: Fix multi-line comment style

  /*
   * Fix multi-line comment style as in this short example. Pay attention
   * to the capitalization, period and starting line of the text.
   */

While at it, split the (supposedly short) description of couple of functions
to summary (short description) and (long) description.
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20211122171721.61553-3-andriy.shevchenko@linux.intel.comSigned-off-by: Mark Brown <broonie@kernel.org>