net/mlx5: Unify and improve command interface

Now as all commands use mlx5 ifc interface, instead of doing two calls
for executing a command we embed command status checking into
mlx5_cmd_exec to simplify the interface.

Also we do here some cleanup for redundant software structures
(inbox/outbox) and functions and improved command failure output.

Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>