net/mlx5: remove redundant size calculation macro

net/mlx5: remove redundant size calculation macro

Patch [1] added, among other definitions, the macro MLX5_ST_SZ_DB.
Patch [2] added later the macro MLX5_ST_SZ_BYTES, which is exactly
the same macro with a different name.
Each of these macros was used in very few places.

This patch removes the definition of MLX5_ST_SZ_DB, and replaces it
with MLX5_ST_SZ_BYTES wherever it was used.
Macro MLX5_ST_SZ_BYTES was preffered since it is the same macro
name used in kernel code, see [3].

[1] http://patches.dpdk.org/patch/45254/
[2] http://patches.dpdk.org/patch/49403/
[3] https://lists.openwall.net/netdev/2014/10/02/75

Signed-off-by: Dekel Peled <dekelp@mellanox.com>
Acked-by: Shahaf Shuler <shahafs@mellanox.com>