sf: unify write enable commands

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

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
diff --git a/drivers/mtd/spi/sst.c b/drivers/mtd/spi/sst.c
index 4dc2db2..6691c1d 100644
--- a/drivers/mtd/spi/sst.c
+++ b/drivers/mtd/spi/sst.c
@@ -96,7 +96,7 @@
 static int
 sst_enable_writing(struct spi_flash *flash)
 {
-	int ret = spi_flash_cmd(flash->spi, CMD_SST_WREN, NULL, 0);
+	int ret = spi_flash_cmd_write_enable(flash);
 	if (ret)
 		debug("SF: Enabling Write failed\n");
 	return ret;