acer-wmi: Respect current backlight level when loading

Set the backlight to use the current brightness when loaded, rather than
always resetting the backlight to maximum brightness.

Fixes kernel bugzilla #14207

Signed-off-by: Carlos Corbacho <carlos@strangeworlds.co.uk>
Reported-by: Denis Mukhin <denis_mukhin@yahoo.com>
Signed-off-by: Len Brown <len.brown@intel.com>