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>