acpi_gpiobus: OR GPIO_PIN_(IN|OUT)PUT into flags

Right now flags is set to 0 before this "=" -> "|=" change, but it will
matter when the NOT_YET section above becomes effective.

MFC after: 2 weeks
Sponsored by: Amazon

(cherry picked from commit c808132731aa999947f4f7810157d7d8d9aaf61e)