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/wbsio_spi.c b/wbsio_spi.c
index 7876198..ded0840 100644
--- a/wbsio_spi.c
+++ b/wbsio_spi.c
@@ -154,7 +154,8 @@
 	if (!mode) {
 		fprintf(stderr, "%s: unsupported command type wr=%d rd=%d\n",
 			__func__, writecnt, readcnt);
-		return 1;
+		/* Command type refers to the number of bytes read/written. */
+		return SPI_INVALID_LENGTH;
 	}
 
 	OUTB(writearr[0], wbsio_spibase);