net/mlx5e: Use net_prefetchw instead of prefetchw in MPWQE TX datapath

Commit e20f0dbf ("net/mlx5e: RX, Add a prefetch command for small
L1_CACHE_BYTES") switched to using net_prefetchw at all places in mlx5e.
In the same time frame, commit 5af75c74 ("net/mlx5e: Enhanced TX
MPWQE for SKBs") added one more usage of prefetchw. When these two
changes were merged, this new occurrence of prefetchw wasn't replaced
with net_prefetchw.

This commit fixes this last occurrence of prefetchw in
mlx5e_tx_mpwqe_session_start, making the same change that was done in
mlx5e_xdp_mpwqe_session_start.

Signed-off-by: Maxim Mikityanskiy <maximmi@mellanox.com>
Reviewed-by: Saeed Mahameed <saeedm@nvidia.com>
Reviewed-by: Tariq Toukan <tariqt@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>