l2tp: Refactor the codes with existing macros instead of literal number

Use PPP_ALLSTATIONS, PPP_UI, and SEND_SHUTDOWN instead of 0xff,
0x03, and 2 separately.

Signed-off-by: Gao Feng <fgao@ikuai8.com>
Signed-off-by: David S. Miller <davem@davemloft.net>