net/mlx5e: Bring back bfreg uar map dedicated pointer

4K Uar series modified the mlx5e driver to use the new bfreg API,
and mistakenly removed the sq->uar_map iomem data path dedicated
pointer, which was meant to be read from xmit path for cache locality
utilization.

Fix that by returning that pointer to the SQ struct.

Fixes: 7309cb4ad71e ("IB/mlx5: Support 4k UAR for libmlx5")
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Reviewed-by: Tariq Toukan <tariqt@mellanox.com>