sf: unify write disable commands

Every spi flash uses the same write disable command, so unify this in
the common code.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Fixed commit message.
Signed-off-by: Wolfgang Denk <wd@denx.de>
diff --git a/drivers/mtd/spi/sst.c b/drivers/mtd/spi/sst.c
index 6691c1d..d1e8a93 100644
--- a/drivers/mtd/spi/sst.c
+++ b/drivers/mtd/spi/sst.c
@@ -105,7 +105,7 @@
 static int
 sst_disable_writing(struct spi_flash *flash)
 {
-	int ret = spi_flash_cmd(flash->spi, CMD_SST_WRDI, NULL, 0);
+	int ret = spi_flash_cmd_write_disable(flash);
 	if (ret)
 		debug("SF: Disabling Write failed\n");
 	return ret;