spi: imx: set spi_bus_clk for mx1, mx31 and mx35

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: Martin Kaiser <martin@kaiser.cx>
Signed-off-by: Mark Brown <broonie@kernel.org>