spi_mcux_dspi: Hotfix for failing null rx test

This is not really high priority to support and is disrupting test
reporting, for now just hotfix the test by returning that this is not
supported, it's not really necessary anyways, there's multiple ways to
call the API to achieve the same effect.

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