Modify spi_imx_clkdiv_2() to return the resulting bus clock frequency
when the selected clock divider is applied. Set spi_imx->spi_bus_clk to
this frequency.

If spi_bus_clk is unset, spi_imx_calculate_timeout() causes a
division by 0.
Signed-off-by: NMartin Kaiser <martin@kaiser.cx>
Signed-off-by: NMark Brown <broonie@kernel.org>