regulator: ab8500-ext: Get rid of is_enabled from struct ab8500_ext_regulator_info

The intention of this patch is to simplify the code.

Maintain the is_enabled flag is not trivial, it not only needs to set/clear the
flag in disable()/enable() but also needs to set the flag in is_enable() to get
initial status. The only benefit of keeping is_enabled flag is just save a
register read when set_mode(). Remove is_enabled flag makes the code simpler.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>