spi_flash: document expected return values

The spi_flash API did not have any of its callbacks
documented. Do that so that people don't have to go
into the guts of an implementation to figure out the
proper expectations.

Change-Id: I55a0515445cab3697813d88373ee413f30b557b5
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: http://review.coreboot.org/10206
Tested-by: build bot (Jenkins)
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>

src/include/spi_flash.h

@@ -42,6 +42,7 @@ struct spi_flash {
 
 	u8		status_cmd;
 
+	/* All callbacks return 0 on success and != 0 on error. */
 	int		(*read)(struct spi_flash *flash, u32 offset,
 				size_t len, void *buf);
 	int		(*write)(struct spi_flash *flash, u32 offset,