regmap: kunit: Fix marking of the range window as volatile

For some reason the regmap used for testing ranges was not including the
end of the range of paged registers as volatile since it found the end by
counting from the selector register rather than the base of the window.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20231023-regmap-test-window-cache-v1-1-d8a71f441968@kernel.org


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