net/mlx5e: Fix wrong (zero) TX drop counter indication for representor

[ Upstream commit 7fdc1adc ]

For representors, the TX dropped counter is not folded from the
per-ring counters. Fix it.

Signed-off-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>