net/mlx5e: Do not request completion on every single UMR WQE

UMR WQEs are posted in bulks, and HW is notified once per a bulk.
Reduce the number of completions by requesting such only for
the last WQE of the bulk.

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