net: bridge: stp: don't cache eth dest pointer before skb pull

Don't cache eth dest pointer before calling pskb_may_pull.

Fixes: cf0f02d0

 ("[BRIDGE]: use llc for receiving STP packets")
Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>