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.com


Signed-off-by: Mark Brown <broonie@kernel.org>