spi: Replace custom fsleep() implementation

_spi_transfer_delay_ns() partially reimplements what fsleep() does.
Replace that code by calling fsleep() instead.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://patch.msgid.link/20250205130624.716039-1-andriy.shevchenko@linux.intel.com


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