iASL: Change result not used warning to an error.

This is the case where an ASL operator is effectively a NOOP because the
result of the operation is not stored anywhere. Example:

Add (4, Local0).

There is no target (3rd argument), nor is the function return value
used.

This is potentially a very serious problem, as the code was probably
intended to do something, but for whatever reason, the value was not
stored.

Therefore, upgrade this issue from a warning to an error.