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

commit 54c151d9ed1321e6e623c80ffe42cd2eb1571744 upstream.

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

Signed-off-by: Gao Feng <fgao@ikuai8.com>
Acked-by: Guillaume Nault <g.nault@alphalink.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Will Deacon <will@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>