net/mlx5e: Check if inner FT is supported outside of create/destroy functions

Move the mlx5e_tunnel_inner_ft_supported() check for inner flow tables
support outside of mlx5e_create_inner_ttc_table() and
mlx5e_destroy_inner_ttc_table(). It allows to avoid accessing invalid
TIRNs of inner indirect TIRs. In a later commit these accesses will be
replaced by getters that will WARN if inner indirect TIRs don't exist.

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