Fix for a Store->ArgX when ArgX contains a reference to a field.

This change fixes a problem where a Store operation to an ArgX object
that contained a reference to a field object did not complete the
automatic dereference and then write to the actual field object.
Instead, the object type of the field object was inadvertently changed
to match the type of the source operand. The new behavior will actually
write to the field object (buffer field or field unit), thus matching
the correct ACPI-defined behavior.