regulator: ab8500: Don't update is_enabled flag in error paths

This avoid setting is_enabled flag to wrong status if enable/disable 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>