net/mlx5: Use u16 for Work Queue buffer strides offset

Minimal stride size is 16.
Hence, the number of strides in a fragment (of PAGE_SIZE)
is <= PAGE_SIZE / 16 <= 4K.

u16 is sufficient to represent this.

Fixes: d7037ad7 ("net/mlx5: Fix QP fragmented buffer allocation")
Signed-off-by: Tariq Toukan <tariqt@mellanox.com>
Reviewed-by: Eran Ben Elisha <eranbe@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>