net/mlx5: Refactor imm_inval_pkey field in cqe struct

The imm_inval_pkey field can hold four different types of data,
depends on the usage, the data could be one of the below:
- Immediate field of the received message
- Invalidate rkey
- Pkey of the packet
- Flow table metadata

Current implementation doesn't reflect the intended usage of the
field at usage time.

Reflect the different types by replace this field with a union,
modify code where this field is used to reflect its intended
usage.

Signed-off-by: Raed Salem <raeds@mellanox.com>
Reviewed-by: Huy Nguyen <huyn@mellanox.com>
Reviewed-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>