cmd_mem: call unmap_sysmem() after map_sysmem()

Actually, unmap_sysmem() does nothing.  Just in case.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Simon Glass <sjg@chromium.org>
Acked-by: Simon Glass <sjg@chromium.org>
diff --git a/common/cmd_mem.c b/common/cmd_mem.c
index bfca59e..0d50dcf 100644
--- a/common/cmd_mem.c
+++ b/common/cmd_mem.c
@@ -480,6 +480,9 @@
 		if ((count % (64 << 10)) == 0)
 			WATCHDOG_RESET();
 	}
+	unmap_sysmem(buf);
+	unmap_sysmem(src);
+
 	return 0;
 }