spi: microchip-core-qspi: set min_speed_hz during probe

The controller's minimum possible bus clock is 1/30 the rate of the
input clock. Naively set the minimum bus clock speed the controller
is capable of during probe, assuming that the rate will never reduce
further.

Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://patch.msgid.link/20250620-drained-widen-ac311bd5f172@spud


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