regulator: ab8500-ext: Don't update info->update_val if set_mode() fails

This ensures info->update_val status is still correct if set_mode() call fails.
Otherwise, get_mode() may return wrong status if a set_mode() call 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>