net/mlx5: Introduce and use opcode getter in command interface

Introduce an opcode getter in the FW command interface, and use it.
Initialize the entry's opcode field early in cmd_alloc_ent() and use it
when possible.

Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
Reviewed-by: Moshe Shemesh <moshe@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>