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

[ Upstream commit 7cb5eb93 ]

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>
Stable-dep-of: 8f5100da ("net/mlx5e: Fix a race in command alloc flow")
Signed-off-by: Sasha Levin <sashal@kernel.org>