dm: spi: prevent setting a speed of 0 Hz

When the device tree is missing a correct spi slave description below
the bus (compatible "spi-flash" or spi-max-frequency are missing),
the 'set_speed' callback can be called with 'speed' == 0 Hz.
At least with cadence qspi, this leads to a division by zero.

Prevent this by initializing speed to 100 kHz in this case (same
fallback value as is done in 'dm_spi_claim_bus') and issue a warning
to console.

Signed-off-by: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>