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>

common/cmd_mem.c

@@ -480,6 +480,9 @@ static int do_mem_cp(cmd_tbl_t *cmdtp, int flag, int argc, char * const argv[])
 		if ((count % (64 << 10)) == 0)
 			WATCHDOG_RESET();
 	}
+	unmap_sysmem(buf);
+	unmap_sysmem(src);
+
 	return 0;
 }