net/mlxfw: Use MLXFW_ERR_MSG macro for error reporting

Instead of always calling both mlxfw_err and NL_SET_ERR_MSG_MOD with the
same message, use the dedicated macro instead.

Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>