spi: bitbang: Use NSEC_PER_*SEC rather than hard coding

Use NSEC_PER_*SEC rather than the hard coded value of 1000s.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20240502154825.2752464-1-andriy.shevchenko@linux.intel.com


Signed-off-by: Mark Brown <broonie@kernel.org>