spi: pxa2xx: Clean up linux/spi/pxa2xx_spi.h

Merge series from Andy Shevchenko <andriy.shevchenko@linux.intel.com>:

A couple of cleanups against linux/spi/pxa2xx_spi.h.

I'm sending this as v3 to land in the SPI subsystem. Meanwhile I'm
preparing an update to make linux/spi/pxa2xx_spi.h private to the
subsystem (PXA2xx driver). But the second part will be presented later
on (likely after v6.9-rc1). That said, this can be routed either via
SoC tree or SPI, up to respective maintainers.