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>

drivers/spi/spi-loopback-test.c

@@ -635,8 +635,8 @@ static int spi_test_check_loopback_result(struct spi_device *spi,
 		} else {
 			/* first byte received */
 			txb = ((u8 *)xfer->rx_buf)[0];
-			/* first byte may be 0 or xff */
-			if (!((txb == 0) || (txb == 0xff))) {
+			/* first byte may be 0 or 0xff */
+			if (txb != 0 && txb != 0xff) {
 				dev_err(&spi->dev,
 					"loopback strangeness - we expect 0x00 or 0xff, but not 0x%02x\n",
 					txb);