spi: pxa2xx: Introduce __lpss_ssp_update_priv() helper

In a few places we repeat RMW IO operations on LPSS private
registers. Let's introduce a helper to make the code better
to read and maintain.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://patch.msgid.link/20250116162109.263081-1-andriy.shevchenko@linux.intel.com


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