net/mlx4_en: Remove selftest TX queues empty condition

Remove waiting for TX queues to become empty during selftest.
This check is not necessary for any purpose, and might put
the driver into an infinite loop.

Signed-off-by: Eugenia Emantayev <eugenia@mellanox.com>
Signed-off-by: Amir Vadai <amirv@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>