isisd: properly display srv6 algorithm

isisd: properly display srv6 algorithm

When the segment-routing ipv6 is configured, the SPF
algoritm shows S-SPF is used:

> rt1# show isis segment-routing srv6 node
> Area 1:
> IS-IS L1 SRv6-Nodes:
>
> System ID Algorithm SRH Max SL SRH Max End Pop SRH Max H.encaps SRH Max End D
> -----------------------------------------------------------------------------------------
> 0000.0000.0001 S-SPF 3 3 2 5

Actually, the segment-routing ipv6 algo capabilities (SPF) are rewritten.
Don't rewrite the algo[0].

> rt1# show isis segment-routing srv6 node
> Area 1:
> IS-IS L1 SRv6-Nodes:
>
> System ID Algorithm SRH Max SL SRH Max End Pop SRH Max H.encaps SRH Max End D
> -----------------------------------------------------------------------------------------
> 0000.0000.0001 SPF 3 3 2 5

Fixes: ("78774bbcd51b") isisd: add isis flex-algo lsp advertisement
Signed-off-by: Dmytro Shytyi <dmytro.shytyi@6wind.com>