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

drivers/pipes/Kconfig

@@ -5,20 +5,23 @@
 
 config DEV_PIPE_MAXSIZE
 	int "Maximum pipe/FIFO size"
-	default 1024
+	default 1024 if !DEFAULT_SMALL
+	default 256 if DEFAULT_SMALL
 	---help---
 		Maximum configurable size of a pipe or FIFO at runtime.
 
 config DEV_PIPE_SIZE
 	int "Default pipe size"
-	default 1024
+	default 1024 if !DEFAULT_SMALL
+	default 256 if DEFAULT_SMALL
 	---help---
 		Sets the default size of the pipe ringbuffer in bytes.  A value of
 		zero disables pipe support.
 
 config DEV_FIFO_SIZE
 	int "Default FIFO size"
-	default 1024
+	default 1024 if !DEFAULT_SMALL
+	default 256 if DEFAULT_SMALL
 	---help---
 		Sets the default size of the FIFO ringbuffer in bytes.  A value of
 		zero disables FIFO support.

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);