mmc: mmc_spi: allow for spi controllers incapable of getting as low as 400k

Some controllers may not be able to reach a bus clock as low as 400 KHz
due to a lack of sufficient divisors. In these cases, the SD card slot
becomes non-functional as Linux continuously attempts to set the bus
clock to 400 KHz. If the controller is incapable of getting that low,
set its minimum frequency instead. While this may eliminate some SD
cards, it allows those capable of operating at the controller's minimum
frequency to be used.

Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20240625-gigantic-frown-1ef4afa3e6fa@wendy


Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>