spi: fsl-dspi: several minor improvements and fixes

 - improve dependencies using COMPILE_TEST
 - fix a typo
 - drop platform_set_drvdata(pdev, NULL) in error path of probe
 - make MODULE_LICENSE match the header

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Mark Brown <broonie@linaro.org>