hsr: fix incorrect warning

When HSR interface is setup using ip link command, an annoying warning
appears with the trace as below:-

[  203.019828] hsr_get_node: Non-HSR frame
[  203.019833] Modules linked in:
[  203.019848] CPU: 0 PID: 158 Comm: sd-resolve Tainted: G        W       4.12.0-rc3-00052-g9fa6bf70 #2
[  203.019853] Hardware name: Generic DRA74X (Flattened Device Tree)
[  203.019869] [<c0110280>] (unwind_backtrace) from [<c010c2f4>] (show_stack+0x10/0x14)
[  203.019880] [<c010c2f4>] (show_stack) from [<c04b9f64>] (dump_stack+0xac/0xe0)
[  203.019894] [<c04b9f64>] (dump_stack) from [<c01374e8>] (__warn+0xd8/0x104)
[  203.019907] [<c01374e8>] (__warn) from [<c0137548>] (warn_slowpath_fmt+0x34/0x44)
root@am57xx-evm:~# [  203.019921] [<c0137548>] (warn_slowpath_fmt) from [<c081126c>] (hsr_get_node+0x148/0x170)
[  203.019932] [<c081126c>] (hsr_get_node) from [<c0814240>] (hsr_forward_skb+0x110/0x7c0)
[  203.019942] [<c0814240>] (hsr_forward_skb) from [<c0811d64>] (hsr_dev_xmit+0x2c/0x34)
[  203.0199...