mlxsw: spectrum_router: Reduce mlxsw_sp_ipip_fib_entry_op_gre4()

Turned out that mlxsw_sp_ipip_fib_entry_op_gre4() does not need to
figure out the IP address and virtual router id. Those are exactly
the same as in the fib_entry it is called for. So just use that and
reduce mlxsw_sp_ipip_fib_entry_op_gre4() function to only call
mlxsw_sp_ipip_fib_entry_op_gre4_rtdp() make the ipip decap op
code similar to nve.

Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>