spi: loopback-test: Simplify strange loopback value check

Apply De Morgan's Theorem and drop superfluous parentheses to simplify
the check for strange loopback values.
While at it, add the missing zero in the related comment.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://patch.msgid.link/da612090f543c8c7cc99fb9dc6ef4abc9560abe4.1746184293.git.geert+renesas@glider.be


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