regulator: max8649: Remove unused check_range() function

This driver has been converted to use regulator_map_voltage_linear and
set_voltage_sel now. regulator_map_voltage_linear will check the the voltage
falls within specified range. The check_range() function is not used, remove it.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>