spi: pxa2xx: derive struct chip_data from struct drv_data

Since we call pxa2xx_ssp_get_clk_div() from pump_transfers() we may derive
pointer to struct chip_data from struct drv_data like it's done in the rest
of the functions. This will make it less errorprone.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>