net/mlx5e: TX, Use correct counter in dma_map error flow

In case of a dma_mapping_error, do not use wi->num_dma
as a parameter for dma unmap function because it's yet
to be set, and holds an out-of-date value.
Use actual value (local variable num_dma) instead.

Fixes: 34802a42 ("net/mlx5e: Do not modify the TX SKB")
Fixes: e586b3b0 ("net/mlx5: Ethernet Datapath files")
Signed-off-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>