Update go-e.go (evcc-io#487)

PR by @tmeidinger

README.md

@@ -156,7 +156,7 @@ In general, due to the minimum value of 5% for signalling the EV duty cycle, the
 Charger is responsible for handling EV state and adjusting charge current. Available charger implementations are:
 
 - `evsewifi`: chargers with SimpleEVSE controllers using [EVSE-WiFi](https://www.evse-wifi.de/)
-- `go-e`: go-eCharger chargers (both local and cloud API are supported)
+- `go-e`: go-eCharger chargers (both local and cloud API are supported, at least firmware 040.0 required)
 - `keba`: KEBA KeContact P20/P30 and BMW chargers (see [Preparation](#keba-preparation))
 - `mcc`: Mobile Charger Connect devices (Audi, Bentley, Porsche)
 - `openWB`: openWB chargers using openWB's MQTT interface

charger/go-e.go

@@ -201,7 +201,7 @@ func (c *GoE) Enable(enable bool) error {
 
 // MaxCurrent implements the Charger.MaxCurrent interface
 func (c *GoE) MaxCurrent(current int64) error {
-	status, err := c.apiUpdate(fmt.Sprintf("amp=%d", current))
+	status, err := c.apiUpdate(fmt.Sprintf("amx=%d", current))
 	if err == nil && isValid(status) && int64(status.Amp) != current {
 		return fmt.Errorf("amp update failed: %d", status.Amp)
 	}