spi: omap2-mcspi: Remove the call to platform_set_drvdata(pdev, NULL)

Remove the call of platform_set_drvdata(pdev, NULL) as they are not
needed anymore.

Signed-off-by: Shubhrajyoti D <shubhrajyoti@ti.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>