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/ramtron.c b/drivers/mtd/spi/ramtron.c
index 078d16c..27d4039 100644
--- a/drivers/mtd/spi/ramtron.c
+++ b/drivers/mtd/spi/ramtron.c
@@ -198,7 +198,7 @@
 
 	if (command == CMD_RAMTRON_WRITE) {
 		/* send WREN */
-		ret = spi_flash_cmd(flash->spi, CMD_RAMTRON_WREN, NULL, 0);
+		ret = spi_flash_cmd_write_enable(flash);
 		if (ret < 0) {
 			debug("SF: Enabling Write failed\n");
 			goto releasebus;