net/mlxfw: Verify FSM error code translation doesn't exceed array size

Array mlxfw_fsm_state_err_str contains value to string translation, when
values are provided by mlxfw_dev. If value is larger than
MLXFW_FSM_STATE_ERR_MAX, return "unknown error" as expected instead of
reading an address than exceed array size.

Fixes: 410ed13c

 ("Add the mlxfw module for Mellanox firmware flash process")
Signed-off-by: Eran Ben Elisha <eranbe@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>