SPI driver: Correct return value in case of a certain error condition

drivers/spi/spi_driver.c

@@ -402,7 +402,7 @@ int spi_register(FAR struct spi_dev_s *spi, int bus)
 
       /* Return the result of the registration */
 
-      return OK;
+      return ret;
     }
 
   return -ENOMEM;

drivers/spi/spi_transfer.c

@@ -99,7 +99,9 @@ int spi_transfer(FAR struct spi_dev_s *spi, FAR struct spi_sequence_s *seq)
   SPI_SETMODE(spi, seq->mode);
   SPI_SETBITS(spi, seq->nbits);
 
-  /* Select the SPI device in preparation for the transfer */
+  /* Select the SPI device in preparation for the transfer.
+   * REVISIT: This is redundant.
+   */
 
   SPI_SELECT(spi, seq->dev, true);