net/mlx5: cmdif, Return value improvements

Make sure that the two basic command interface functions cmd_exec and
cmd_invoke will return well defined return values:

return < 0 : Command execution couldn't be submitted by driver
return > 0 : Command execution couldn't be performed by firmware
return = 0 : Command was executed by FW, Caller must check FW
outbox status.

These statuses are valid for the blocking call of cmd_exec() e.g. when
callback == NULL, in a downstream patch, will refactor the code to
provide the same return value semantics to the callback.

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