The modpost script is not happy

  WARNING: modpost: missing MODULE_DESCRIPTION() in drivers/spi/spi-bitbang.o

because there is a missing module description.

Add it to the module.

While at it, update the terminology in Kconfig section to be in align
with added description along with the code comments.
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20240502171518.2792895-1-andriy.shevchenko@linux.intel.comSigned-off-by: Mark Brown <broonie@kernel.org>