spi: bcm2835: Set controller max_speed_hz

Set the struct spi_controller max_speed_hz. This is based on the
reported source clock frequency during probe. The maximum bus clock
is half the source clock (as per the code in bcm2835_spi_transfer_one).
Signed-off-by: Richard Fitzgerald <rf@opensource.cirrus.com>
Link: https://lore.kernel.org/r/20210107164825.21919-1-rf@opensource.cirrus.comSigned-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi-bcm2835.c

@@ -1307,6 +1307,8 @@ static int bcm2835_spi_probe(struct platform_device *pdev)
 		return dev_err_probe(&pdev->dev, PTR_ERR(bs->clk),
 				     "could not get clk\n");
 
+	ctlr->max_speed_hz = clk_get_rate(bs->clk) / 2;
+
 	bs->irq = platform_get_irq(pdev, 0);
 	if (bs->irq <= 0)
 		return bs->irq ? bs->irq : -ENODEV;