spi: rspi: Add spi_master_get() call to prevent use after free

In rspi_remove(), current code dereferences rspi after spi_unregister_master(),
thus add an extra spi_master_get() call is necessary to prevent use after free.

Current code already has an extra spi_master_put() call in rspi_remove(), so
this patch just adds a spi_master_get() call rather than a spi_master_get() with
spi_master_put() calls.
Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Mark Brown <broonie@linaro.org>

drivers/spi/spi-rspi.c

@@ -726,7 +726,7 @@ static void rspi_release_dma(struct rspi_data *rspi)
 
 static int rspi_remove(struct platform_device *pdev)
 {
-	struct rspi_data *rspi = platform_get_drvdata(pdev);
+	struct rspi_data *rspi = spi_master_get(platform_get_drvdata(pdev));
 
 	spi_unregister_master(rspi->master);
 	rspi_release_dma(rspi);