Replace sony_acpi_value.{min,max} with a callback function that allows
more complex reasoning in accepting input and presenting output.

This allows consistency between the sony-laptop specific 'brightness_default'
and the backlight subsystem 0-based 'brightness'.
Signed-off-by: NMattia Dongili <malattia@linux.it>
Signed-off-by: NLen Brown <len.brown@intel.com>