mlxsw: spectrum_router: Re-order mlxsw_sp_nexthop6_group_get()

Attach the FIB entry to the nexthop group after setting the offload flag
on the IPv6 FIB info (i.e., 'struct fib6_info'). The second operation is
not needed when the nexthop group is a nexthop object. This will allow
us to have a common exit path from the function, regardless of the
nexthop group's type.

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