spi: mtk_spim: get spi clk rate only once

We don't really need to switch clk rate during operating SPIM controller.
Get clk rate only once at driver probing.

Signed-off-by: SkyLake.Huang <skylake.huang@mediatek.com>
Signed-off-by: Weijie Gao <weijie.gao@mediatek.com>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>