[PATCH] SPI: busnum == 0 needs to work

We need to be able to have a "SPI bus 0" matching chip numbering; but
that number was wrongly used to flag dynamic allocation of a bus number.

This patch resolves that issue; now negative numbers trigger dynamic alloc.

It also updates the how-to-write-a-controller-driver overview to mention
this stuff.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

include/linux/spi/spi.h

@@ -172,13 +172,13 @@ static inline void spi_unregister_driver(struct spi_driver *sdrv)
 struct spi_master {
 	struct class_device	cdev;
 
-	/* other than zero (== assign one dynamically), bus_num is fully
+	/* other than negative (== assign one dynamically), bus_num is fully
 	 * board-specific.  usually that simplifies to being SOC-specific.
-	 * example:  one SOC has three SPI controllers, numbered 1..3,
+	 * example:  one SOC has three SPI controllers, numbered 0..2,
 	 * and one board's schematics might show it using SPI-2.  software
 	 * would normally use bus_num=2 for that controller.
 	 */
-	u16			bus_num;
+	s16			bus_num;
 
 	/* chipselects will be integral to many controllers; some others
 	 * might use board-specific GPIOs.