Use a distinct return code for SPI commands with unsupported/invalid length

Some drivers support only a few combinations of read/write length and
return error otherwise. Having a distinct return code for this error
means we can handle it in upper layers.

Corresponding to flashrom svn r653.

Signed-off-by: Carl-Daniel Hailfinger <c-d.hailfinger.devel.2006@gmx.net>
Acked-by: Stefan Reinauer <stepan@coresystems.de>
diff --git a/spi.h b/spi.h
index aa80c43..3050ccf 100644
--- a/spi.h
+++ b/spi.h
@@ -108,5 +108,6 @@
 /* Error codes */
 #define SPI_INVALID_OPCODE	-2
 #define SPI_INVALID_ADDRESS	-3
+#define SPI_INVALID_LENGTH	-4
 
 #endif		/* !__SPI_H__ */