drivers: regulator: fixed: refactor initialization code.

In some cases, the enable pin may be already enabled by a previous
stage, e.g. bootloader. Therefore, it is not desirable to disable
the pin, as it could cause malfunctioning of the device. Refactor init
procedure so that we pick the right GPIO flags during the first
configuration stage.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>