gpiopower: Fix failure case for attach

Summary:
We really should be checking the return value of
ofw_gpiobus_parse_gpios_impl, not the value of sc_pins, which isn't
changed on failure.

Reported by: alfredo@

Reviewers: PowerPC, alfredo

Reviewed By: PowerPC, alfredo

Subscribers: imp, loos

Differential Revision: https://reviews.freebsd.org/D34571