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>
diff --git a/drivers/spi/spi.c b/drivers/spi/spi.c
index 978dda2..7e3446c 100644
--- a/drivers/spi/spi.c
+++ b/drivers/spi/spi.c
@@ -1375,6 +1375,7 @@
 	 * it is not set for this transfer.
 	 */
 	list_for_each_entry(xfer, &message->transfers, transfer_list) {
+		message->frame_length += xfer->len;
 		if (!xfer->bits_per_word)
 			xfer->bits_per_word = spi->bits_per_word;
 		if (!xfer->speed_hz)