regulator: rpi-panel-attiny: use new GPIO line value

Merge series from Bartosz Golaszewski <brgl@bgdev.pl>:

struct gpio_chip now has callbacks for setting line values that return
an integer, allowing to indicate failures. We're in the process of
converting all GPIO drivers to using the new API. This series converts
the only GPIO controller under drivers/regulator/ and - while at it -
refactors the code a bit.