raspberry_virtgpio: fix OF_hasprop() usage

The function returns a bool. This driver was merged recently (by me) and
I missed this instance.

While here, adjust the ofw_bus_status_okay() call similarly. This
function still returns an int, but this usage is more widely used in our
drivers.

No functional change intended.

MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Fixes: b60cd486a652 ("ofw: bool-ify OF_hasprop()")