WCN6855: add symbolic link for hw2.1

At least at the moment WCN6855 hw2.1 and hw2.0 share the same firmware files,
so add symlink pointing from hw2.1 to hw2.0.

diff --git a/WCN6855/hw2.1 b/WCN6855/hw2.1
new file mode 120000 (symlink)
index 0000000..e10f8a2
--- /dev/null
+++ b/WCN6855/hw2.1
@@ -0,0 +1 @@
+hw2.0
\ No newline at end of file