net/mlx4: small optimization in mlx4_en_xmit()

Test against MLX4_MAX_DESC_TXBBS only matters if the TX
bounce buffer is going to be used.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Wei Wang <weiwan@google.com>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>