spi: document odd controller reference handling

Document the fact that a reference to the controller is dropped as part
of deregistration.

This is an odd pattern as the reference is typically taken in
__spi_alloc_controller() rather than spi_register_controller(). Most
controller drivers gets it right these days and notably the
device-managed interface relies on this behaviour.
Signed-off-by: NJohan Hovold <johan@kernel.org>
Signed-off-by: NMark Brown <broonie@kernel.org>

drivers/spi/spi.c

@@ -2195,7 +2195,7 @@ static void devm_spi_unregister(struct device *dev, void *res)
  * Context: can sleep
  *
  * Register a SPI device as with spi_register_controller() which will
- * automatically be unregister
+ * automatically be unregistered and freed.
  *
  * Return: zero on success, else a negative error code.
  */
@@ -2236,6 +2236,8 @@ static int __unregister(struct device *dev, void *null)
  * only ones directly touching chip registers.
  *
  * This must be called from context that can sleep.
+ *
+ * Note that this function also drops a reference to the controller.
  */
 void spi_unregister_controller(struct spi_controller *ctlr)
 {