spi: rcar: Fix pointer cast in the remove function

The platform driver data is set to point to the rspi_data structure at
probe time. Calling spi_master_get() on the pointer is just plain wrong
and only works by chance. Fix it by using the platform driver data
directly.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Mark Brown <broonie@linaro.org>