gpioc: Fix handling of priv data during open

Fix the ordering of priv data creation with setting priv data. This
handles failure better and resolves a panic when repeatedly running
tools/tools/gpioevents.

Explicitly initialise more fields in priv data while we are here.

Reviewed by: markj
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D46568