framebufferio: Fix crash calling unimplemented get_brightness
framebufferio: Fix crash calling unimplemented get_brightness

If set_brightness was implemented but get_brightness wasn't, this
would hard fault

adafruit/circuitpython,adafruit/circuitpython,adafruit/circuitpython,adafruit/circuitpython,adafruit/circuitpython,adafruit/circuitpython