net/mlx5e: TX, Move DB fields in TXQ-SQ struct

Pointers in DB are static, move them to read-only area so they
do not share a cacheline with fields modified in datapath.

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