Fix two signedness warnings

This helps towards #1792

Author: matthijskooijman

Diff for: hardware/arduino/sam/cores/arduino/wiring_analog.c

@@ -150,7 +150,7 @@ uint32_t analogRead(uint32_t ulPin)
 			// Enable the corresponding channel
 			if (ulChannel != latestSelectedChannel) {
 				adc_enable_channel( ADC, ulChannel );
-				if ( latestSelectedChannel != -1 )
+				if ( latestSelectedChannel != (uint32_t)-1 )
 					adc_disable_channel( ADC, latestSelectedChannel );
 				latestSelectedChannel = ulChannel;
 			}

Diff for: hardware/arduino/sam/variants/arduino_due_x/variant.cpp

@@ -380,7 +380,7 @@ void init( void )
   __libc_init_array();
 
   // Disable pull-up on every pin
-  for (int i = 0; i < PINS_COUNT; i++)
+  for (unsigned i = 0; i < PINS_COUNT; i++)
 	  digitalWrite(i, LOW);
 
   // Enable parallel access on PIO output data registers