spi: pxa2xx: shift clk_div in one place

This patch refactors ssp_get_clk_div() and pxa2xx_ssp_get_clk_div() to align
clk_div calculations, i.e. ssp_get_clk_div() and quark_x1000_set_clk_regvals()
will return plain clk_div and it will be shifted to proper position in
pxa2xx_ssp_get_clk_div().

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