net/mlx5e: Tx, Make SQ WQE fetch function type generic

Change mlx5e_sq_fetch_wqe to be agnostic to the Work Queue
Element (WQE) type.
Before this patch, it was specific for struct mlx5e_tx_wqe.

In order to allow the change, the function now returns the
generic void pointer, and gets the WQE size to do the zero
memset.

Signed-off-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: Eran Ben Elisha <eranbe@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>