ACPI: utils: Rearrange in acpi_evaluate_reference()

The code in acpi_evaluate_reference() can be improved in some ways
without changing its observable behavior.  Among other things:

 * None of the local variables in that function except for buffer
   needs to be initialized.

 * The element local variable is only used in the for () loop block,
   so it can be defined there.

 * Multiple checks can be combined.

 * Code duplication related to error handling can be eliminated.

 * Redundant inner parens can be dropped.

Modify the function as per the above.

No intentional functional impact.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>