firmware: cs_dsp: Add some sanity-checking to test harness

Add sanity checking to some test harness functions to help catch bugs
in the test code. This consists of checking the range of some arguments
and checking that reads from the dummy regmap succeed.

Most of the harness code already had sanity-checking but there were a
few places where it was missing or was assumed that the test could be
trusted to pass valid values.

Signed-off-by: Richard Fitzgerald <rf@opensource.cirrus.com>
Link: https://patch.msgid.link/20250416122422.783215-1-rf@opensource.cirrus.com


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