backlight: pwm_bl: Don't assign levels table repeatedly

pwm_backlight_probe() re-assigns pb->levels for every brightness
level. This is not needed and was likely not intended, since
neither side of the assignment changes during the loop. Assign
the field only once.

Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>