spi: spi-fsl-dspi: Avoid NULL pointer in dspi_slave_abort for non-DMA mode

spi: spi-fsl-dspi: Avoid NULL pointer in dspi_slave_abort for non-DMA mode

The driver does not create the dspi->dma structure unless operating in
DSPI_DMA_MODE, so it makes sense to check for that.

Fixes: f4b323905d8b ("spi: Introduce dspi_slave_abort() function for NXP's dspi SPI driver")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Tested-by: Michael Walle <michael@walle.cc>
Link: https://lore.kernel.org/r/20200318001603.9650-8-olteanv@gmail.com
Signed-off-by: Mark Brown <broonie@kernel.org>