esp32-s2: Don't set PWMOut frequency to 0

FeatherS2 crashes if you set the PWMOut frequency to 0.
This change will raise `ValueError: Invalid PWM frequency` if the requested frequency is 0.
(Lifted from the atmel-samd port)

ports/esp32s2/common-hal/pwmio/PWMOut.c

@@ -129,6 +129,12 @@ pwmout_result_t common_hal_pwmio_pwmout_construct(pwmio_pwmout_obj_t *self,
         return PWMOUT_INITIALIZATION_ERROR;
     }
     
+    // check the frequency -- 0 is not valid
+    // maybe also want to reject frequency > system_clock / 2 ??
+    if (frequency == 0) {
+        return PWMOUT_INVALID_FREQUENCY;
+    }
+    
     // Make reservations
     reserved_timer_freq[timer_index] = frequency;
     reserved_channels[channel_index] = timer_index;