[flang][openacc] Restrict number of device_type values on the set directive

The standard suggests that the value for the `device_type` clause on the
`set` directive is a list but this does not makes sense. Restrict the number
of value to one so it matches the runtime function.

Depends on D158644

Reviewed By: razvanlupusoru

Differential Revision: https://reviews.llvm.org/D158645