net/mlx5e: TLS, Read capabilities only when it is safe

Read TLS caps from the core driver only when TLS is supported, i.e
mlx5_accel_is_tls_device returns true.

Fixes: 790af90c ("net/mlx5e: TLS, build TLS netdev from capabilities")
Change-Id: I5f21ff4d684901af487e366a7e0cf032b54ee9cf
Reported-by: Michal Kubecek <mkubecek@suse.cz>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Reviewed-by: Boris Pismenny <borisp@mellanox.com>
Reviewed-by: Tariq Toukan <tariqt@mellanox.com>