ledc.c: Fix analogWrite() last channel available verification (#8509)

* Fix analogWrite channel available verification

The last channel allocated is number 0, which conflicted with the value given to an uninitialized pin, giving the "No more analogWrite channels available!" error when trying to use it
Pins are now given the value -1 to indicate that they are not used so channel 0 can be used without errors.

* Fix incorrect array initialization

Keeping array of zeros for `pin_to_channel` and shifting stored channel
values by +1 to keep the pin with channel 0 from being interpreted as unused.

ref: #8509 (comment)

Author: rlipas

cores/esp32/esp32-hal-ledc.c

@@ -234,13 +234,13 @@ void analogWrite(uint8_t pin, int value) {
             return;
         }
         ledcAttachPin(pin, channel);
-        pin_to_channel[pin] = channel;
+        pin_to_channel[pin] = channel + 1;
         ledcWrite(channel, value);
     }
 }
 
 int8_t analogGetChannel(uint8_t pin) {
-    return pin_to_channel[pin];
+    return pin_to_channel[pin] - 1;
 }
 
 void analogWriteFrequency(uint32_t freq) {