spi: spidev_test: Use %u to format unsigned numbers

Consistently use %u to format unsigned numbers.
For "bits" this doesn't matter that much, as it is "uint8_t".
However, "speed" is "uint32_t", so in case people use "-s -1" to force
the maximum, they would see:

    max speed: -1 Hz (4294967 KHz)

While at it, use "k" (kilo) instead of "K" (kelvin) in "kHz".

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20200608100049.30648-1-geert+renesas@glider.be


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