Loadpoint: use measured phases/currents as fallback to api.PhaseGetter, api.GetMaxCurrent

[andig]

Fix #14443

@GrimmiMeloni the change is two-fold:

General:

• extract the common && enabled condition to the switch (its obvious from the diff that this can be hoisted)

Phases:

• only evaluate if shouldBeConsistent && phases > 0
• use measuredPhases as go-in value, corrected for 2 active phases

Currents:

• use measured currents instead of set current as fallback

wdyt?

/cc @MarkusGH

[GrimmiMeloni]

Looks good to me. See detailed question around chargerPhases > 0 - but it is more for my understanding.