leds-gpio: Remove stray assignment to brightness_set

The brightness_set method is intended for use cases that must not
block, and can only be used if the GPIO provider can never sleep.
Remove an accidental initialisation (a copy-and-paste error) that
sets it regardless, which has been seen to cause crashes with the
gpio expander driver.

Signed-off-by: Phil Elwell <phil@raspberrypi.org>