tests: spi_loopback: skip hold_on_cs test if not supported

This early exit section is only reached if there is an error.
In the case of not supported, the ret variable will be 0 and zassert
won't happen. This is because the test was meant to be skipped, not
passed or failed, in this case. So add the skip call.

Signed-off-by: Declan Snyder <declan.snyder@nxp.com>

Author: decsny

tests/drivers/spi/spi_loopback/src/spi.c

@@ -1017,6 +1017,8 @@ ZTEST(spi_extra_api_features, test_spi_hold_on_cs)
 early_exit:
 	hold_spec->config.operation &= ~SPI_HOLD_ON_CS;
 	zassert_false(ret, "SPI transceive failed, code %d", ret);
+	/* if there was no error then it was meant to be a skip at this point */
+	ztest_test_skip();
 }
 
 /*