Documentation: xfrm_device: Use numbered list for offloading steps

Format xfrm offloading steps as numbered list.

Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>