spi: pxa2xx: Constify struct driver_data parameter

In a couple of functions the contents of struct driver_data are not altered,
hence we may constify the respective function parameter.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20210510124134.24638-14-andriy.shevchenko@linux.intel.com


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