spi: imx: set spi_bus_clk for mx21 and mx27

Introduce additional output parameter in spi_imx_clkdiv_1()
function to return result frequency and set it to spi_bus_clk.

This fixes division by zero bug, which occurred in
spi_imx_calculate_timeout() function.

Signed-off-by: Robert Baldyga <r.baldyga@hackerion.com>
Signed-off-by: Mark Brown <broonie@kernel.org>