net/mlx5e: Define one flow for TXQ selection when TCs are configured

We shall always extract channel index out of the txq, regardless
of the relation between txq_ix and num channels. The extraction is
always valid, as if txq is smaller than number of channels,
txq_ix == priv->txq2sq[txq_ix]->ch_ix.

By doing so, we can remove an if clause from the select queue method,
and have one flow for all packets.

Signed-off-by: Eran Ben Elisha <eranbe@mellanox.com>
Reviewed-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>