spi: spi-fsl-dspi: Reduce indentation in dspi_release_dma()

There is no point in surrounding an entire function block in an if
condition. Rather, exit early if the condition is false.

Signed-off-by: Vladimir Oltean <olteanv@gmail.com>
Link: https://lore.kernel.org/r/20190818180115.31114-7-olteanv@gmail.com
Signed-off-by: Mark Brown <broonie@kernel.org>