regulator: ab8500-ext: Remove enable() and disable() functions

Both enable() and disable() functions have only one caller, thus remove them.

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>