regulator: Convert max8997 to get_voltage_sel

Also rename get_current_limit and set_current_limit callbacks
to max8997_get_current_limit and max8997_set_current_limit for
better readability.

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