net/mlx5e: kTLS, Use kernel API to extract private offload context

Modify the implementation of the private kTLS TX HW offload context
getter and setter, so it uses the kernel API functions, instead of
a local shadow structure.
A single BUILD_BUG_ON check is sufficient, remove the duplicate.

Signed-off-by: Tariq Toukan <tariqt@mellanox.com>
Reviewed-by: Maxim Mikityanskiy <maximmi@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>