regulator: ab8500-ext: Make the return type of ab8500_ext_regulator_exit() void

ab8500_ext_regulator_exit() never fails.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Bengt Jonsson <bengt.g.jonsson@stericsson.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>