regulator: ab8500: Remove unused voltage_shift field from struct expand_register

The voltage_shift field of struct expand_register is not used now, remove it.

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>