gpiopps: don't use internal gpiobus function

gpiobus_release_pin is only meant to be used internally by gpiobus. Use
gpio_pin_release instead.

This also fixes a memory leak since gpio_pin_get_by_ofw_idx returns a
malloc'd pointer, which gpio_pin_release now frees.

Reviewed by: wulf, imp
Approved by: imp (mentor)
Differential Revision: https://reviews.freebsd.org/D50871