net/mlx5: Fix size field in bufferx_reg struct

According to HW spec the field "size" should be 16 bits
in bufferx register.

Fixes: e281682b ("net/mlx5_core: HW data structs/types definitions cleanup")
Signed-off-by: Mohammad Kabat <mohammadkab@nvidia.com>
Reviewed-by: Moshe Shemesh <moshe@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>