regulator: ab8500: Don't update info->update_val if write to register fails

This patch ensures info->update_val is consistent with current register value.

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>