mlxsw: spectrum_router: Only clear offload indication from valid IPv6 FIB info

When the IPv6 FIB info has a nexthop object, the nexthop offload
indication is set on the nexthop object and not on the FIB info itself.

Therefore, do not try to clear the offload indication from the FIB info
when it has a nexthop object.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>