spi: spidev_test: add support for word delay

Support setting the word delay using the -w/--word-delay command line
parameter. Note that spidev exposes word delay only as an u8, allowing
for a maximum of 255us of delay to be inserted.

Signed-off-by: Jonas Rebmann <jre@pengutronix.de>
Link: https://patch.msgid.link/20241107-spidev-test-word-delay-v1-1-d4bba5569e39@pengutronix.de


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