driver: spi: Modify core to compute the message length

Make spi core calculate the message length while
populating the other transfer parameters.

Usecase, driver can use it to populate framelength filed in their
controller.

Signed-off-by: Sourav Poddar <sourav.poddar@ti.com>
Signed-off-by: Mark Brown <broonie@linaro.org>

include/linux/spi/spi.h

@@ -578,6 +578,7 @@ struct spi_message {
 	/* completion is reported through a callback */
 	void			(*complete)(void *context);
 	void			*context;
+	unsigned		frame_length;
 	unsigned		actual_length;
 	int			status;