spi: pxa2xx-pci: Extract pxa2xx_spi_pci_clk_register()

Extract pxa2xx_spi_pci_clk_register() from ->probe() in order to reuse it
later on for getting rid of max_clk_rate temporary storage.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20220225172350.69797-9-andriy.shevchenko@linux.intel.com


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