greybus: spi: validate spi master register

Check for error in registering spi master, even though the current code
will fail a little more ahead when trying to register devices in the
master.
Signed-off-by: Rui Miguel Silva <rui.silva@linaro.org>
Reported-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>

drivers/staging/greybus/spi.c

@@ -360,6 +360,8 @@ static int gb_spi_connection_init(struct gb_connection *connection)
 	master->transfer_one_message = gb_spi_transfer_one_message;
 
 	ret = spi_register_master(master);
+	if (ret < 0)
+		goto out_put_master;
 
 	/* now, fetch the devices configuration */
 	for (i = 0; i < spi->num_chipselect; i++) {