net/mlx5e: xsk: Remove dead code in validation

One of the checks in mlx5e_rx_is_linear_skb verifies that the RX buffer
fits into the XSK frame size. Remove the duplicating check from
mlx5e_validate_xsk_param. It allows to make mlx5e_rx_get_min_frag_sz
static.

Remove mlx5e_rx_is_xdp altogether, as its only usage is located in a
branch where xsk == NULL.

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