xfrm: Remove documentation WARN_ON to limit return values for offloaded SA

The original idea to put WARN_ON() on return value from driver code was
to make sure that packet offload doesn't have silent fallback to
SW implementation, like crypto offload has.

In reality, this is not needed as all *swan implementations followed
this request and used explicit configuration style to make sure that
"users will get what they ask".
So instead of forcing drivers to make sure that even their internal flows
don't return -EOPNOTSUPP, let's remove this WARN_ON.

Signed-off-by: Patrisious Haddad <phaddad@nvidia.com>
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>