spi: simplify spi_sync() calling convention

Simplify spi_sync calling convention, eliminating the need to check both
the return value AND the message->status.  In consequence, this corrects
misbehaviours of spi_read and spi_write (which only checked the former) and
their callers.
Signed-off-by: Marc Pignat <marc.pignat@hevs.ch>
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/spi/spi.c

@@ -541,10 +541,7 @@ static void spi_complete(void *arg)
  * Also, the caller is guaranteeing that the memory associated with the
  * message will not be freed before this call returns.
  *
- * The return value is a negative error code if the message could not be
- * submitted, else zero.  When the value is zero, then message->status is
- * also defined;  it's the completion code for the transfer, either zero
- * or a negative error code from the controller driver.
+ * It returns zero on success, else a negative error code.
  */
 int spi_sync(struct spi_device *spi, struct spi_message *message)
 {
@@ -554,8 +551,10 @@ int spi_sync(struct spi_device *spi, struct spi_message *message)
 	message->complete = spi_complete;
 	message->context = &done;
 	status = spi_async(spi, message);
-	if (status == 0)
+	if (status == 0) {
 		wait_for_completion(&done);
+		status = message->status;
+	}
 	message->context = NULL;
 	return status;
 }
@@ -628,10 +627,8 @@ int spi_write_then_read(struct spi_device *spi,
 
 	/* do the i/o */
 	status = spi_sync(spi, &message);
-	if (status == 0) {
+	if (status == 0)
 		memcpy(rxbuf, x[1].rx_buf, n_rx);
-		status = message.status;
-	}
 
 	if (x[0].tx_buf == buf)
 		mutex_unlock(&lock);