ADC: on channel overflow we should not zero last_ch6_micros

this happens every 64 seconds because of unused channels on the
ADC. Zeroing this creates a bad delta_t value for the DCM code.

libraries/AP_ADC/AP_ADC_ADS7844.cpp

@@ -116,7 +116,6 @@ void AP_ADC_ADS7844::read(uint32_t tnow)
 			// reader below could get a division by zero
 			_sum[ch] = 0;
 			_count[ch] = 1;
-			last_ch6_micros = tnow;
 		}
 		_sum[ch] += (v >> 3);
 	}